Author Topic: Questions about replacing oxygen sensors on my car  (Read 874 times)

secondcor521

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Questions about replacing oxygen sensors on my car
« on: September 24, 2020, 08:59:04 PM »
1993 Lexus GS300.  187K miles and I'm pretty sure the oxygen sensors have never been replaced.  Online sources indicate they are a wear item and should be replaced every 60K to 90K miles.  Car is running very rich periodically.  No trouble codes thrown as far as I know (CEL is off).  No other issues with the car.

So I'm thinking about replacing them because it might make the problem go away.

Would you replace all three (because they're all probably 27 years old), or just the two upstream (because that's apparently what affects the fuel mixture, and let downstream sleeping dogs lie)?

Ripple4

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Re: Questions about replacing oxygen sensors on my car
« Reply #1 on: September 25, 2020, 06:10:26 AM »
I think if it was the case that a O2 sensor was bad, sometime in the past it might have create a diagnostic trouble code that may or may not have made the malfunction indicator light come on. so checking for trouble codes at auto zone or buying your own Bluetooth ELM327 dongle will tell you this.

on an old car like this there is the possibility of end of life failure issues coming up on almost every part of the car, so it could be costly start swapping out parts. For myself, in the past, once I spend some money then the sunk cost fallacy kicks in and will lead to even more spending to make the already spent money 'less' wasted.

chemistk

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Re: Questions about replacing oxygen sensors on my car
« Reply #2 on: September 25, 2020, 06:22:50 AM »
Most of the time, a failed O2 sensor will trigger a code. It doesn't always happen, but most of the time it will.

The trouble with O2 sensors is that, assuming you want to DIY, they can be a royal PITA to remove. Since they are mounted directly to the exhaust, the hot/cold cycles typically make them one of the hardest things to get unstuck. Worse, some cars (I have not looked at your particular configuration) don't have much space to allow you to efficiently remove them.

You could absolutely attempt to swap them, but make sure you have a quality penetrating oil, breaker bar, and ideally an O2 sensor socket*. It's not a guarantee that it's going to solve your problem. Other reasons you're running rich could be a failing MAF sensor, worn spark plugs, or failing thermostat. O2 sensors are the first choice to address your issue, but not guaranteed to solve the issue.

*You don't necessarily need the special socket, but it's a major help. You'll be relegated to using box-end wrenches or cutting the old sensor wires to fit a proper socket. If you cut the wires, you're pretty much stuck until you can get the old ones out and the new ones in. If you decide to spring for the O2 sensor socket, make sure you get a stubby one and not one that looks like a spark plug socket with a slot cut out - those will flex and probably break off before you get the sensor broken free.

Uturn

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Re: Questions about replacing oxygen sensors on my car
« Reply #3 on: September 25, 2020, 06:38:56 AM »
OBDII was not a requirement until 1996.  There is a good chance that your car has OBDI and does not have to code you expect to see.
You might be able to borrow the O2 sensor socket from the parts store.

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Re: Questions about replacing oxygen sensors on my car
« Reply #4 on: September 25, 2020, 10:07:27 AM »
The downstream sensor is just for cat converter efficiency.  If you're not getting a code, it's doing what it needs to be doing.  Waste of time and money to replace.

For upstream, if you decide to replace, get the O2 socket on one of the sensors.  Then run the car for 10 minutes to heat up the exhaust.  Then put on your ratchet and easily unscrew the sensor.  Wait for cool down to install the new one unless you like avoiding things that can give you 3rd degree burns.

I've done many of these that cold, they're near impossible to move.  Hot, they're easier than anything.

secondcor521

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Re: Questions about replacing oxygen sensors on my car
« Reply #5 on: September 25, 2020, 11:40:31 AM »
Thanks all for the various feedback and suggestions, they're all very helpful.

@Ripple4, your second paragraph was very helpful.  In thinking about it this morning, I was frustrated that this problem had recurred (car was doing the same thing back in February; removing the negative battery cable made it go away for about seven months) and was just grasping at straws rather than actually diagnosing the problem.  Which is ironic, because I had an auto shop do that to me on this car, where they were guessing that it had a bad head gasket and I should just throw the car in the trash; another shop diagnosed it properly as a radiator issue which was like $200 and fixed that problem.

I checked the OBD codes just now.  It's an old car, so you have to jumper a diagnostic port and then the CEL does a sort of Morse code thing.  It shows a 41 code, which indicates trouble in the throttle position sensor, which the computer uses to set the fuel mixture, so that (a) tracks with the symptom, and (b) makes me think I should skip the apparently functioning O2 sensors for now.  In the spring it was also showing a 24 code, which is the air intake temperature sensor, but I don't know how old that code was.  I *do* know that the 41 code was sometime between February and now, and since that matches my symptom I think I'll try to go down figuring out how to fix (or pay someone to fix) the TPS circuit.

I found this that seems to be a pretty good resource for my car's issue:

https://manualzz.com/doc/23692662/diag.-code-41--47--sub%E2%80%93--throttle-position-sensor---lexus

secondcor521

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Re: Questions about replacing oxygen sensors on my car
« Reply #6 on: September 25, 2020, 12:08:39 PM »
I guess the next question, for those who have more experience with cars...

I understand that the code 41 means that there is a problem somewhere in the TPS circuit.  I've read a bit about this online, and apparently you can replace the TPS itself.  I've also read about fixing the wiring or the harness.

If I just replace the TPS or pay someone to replace the TPS, is that something that would likely fix this kind of issue?  Or is it more likely that a wire is loose or flakey somewhere and that problem needs to be addressed?

I'm inclined to think that the TPS itself is probably fine, and that it is a loose / bad / old / wire that is failing.  I'm also inclined to think that if I go to someone and say, "trouble code 41" that their inclination will be to replace the TPS rather than do what I would imagine to be fiddly work of checking wires and connections and voltages and stuff.  I'm also inclined to think that I'm not sure I could find a good mechanic who is willing to do the fiddly work rather than just replace-and-hope.

mntnmn117

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Re: Questions about replacing oxygen sensors on my car
« Reply #7 on: September 25, 2020, 03:55:41 PM »
I would replace the TPS. On an old vehicle like that there are moving parts inside the sensor. Contact points that physically wear out.  That being on the top of the engine will be a quick easy swap.

I had a 1990 BMW that I was able to just open up, clean and dielectric grease back to function but that just delays the inevitable.

secondcor521

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Re: Questions about replacing oxygen sensors on my car
« Reply #8 on: September 25, 2020, 05:22:58 PM »
Thanks, that's the way I'm leaning.  I figure the code has narrowed it down to two problems - either sensor or wiring.  Replace the sensor myself for $50 or so - the replacement looks easy enough, and I know how to clear the code.  Maybe I get lucky and the problem is resolved.  If not, I pretty much know it's a wiring issue.

The car is 27 years old.  I figure $50 every 27 years for a TPS sensor is acceptable.

(I did see about rotating the TPS sensor to adjust the idle until it's smooth.  I think I'll mark or note where my existing one is and then try to put the replacement one in at the same angle.)

Syonyk

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Re: Questions about replacing oxygen sensors on my car
« Reply #9 on: September 25, 2020, 08:38:34 PM »
1993 Lexus GS300.  187K miles and I'm pretty sure the oxygen sensors have never been replaced.  Online sources indicate they are a wear item and should be replaced every 60K to 90K miles.  Car is running very rich periodically.  No trouble codes thrown as far as I know (CEL is off).  No other issues with the car.

They're definitely wear items, and that old, they're not the heated/wideband kind (almost certainly the single wire sort).  I like to replace them every 80k miles or so, though when I was running a fleet of used vehicles of substantial age/mileage, I just replaced them when I got the car and did the general tuneup (plugs, wires, cap, rotor, oxygen sensor, filters, etc).

I think if it was the case that a O2 sensor was bad, sometime in the past it might have create a diagnostic trouble code that may or may not have made the malfunction indicator light come on. so checking for trouble codes at auto zone or buying your own Bluetooth ELM327 dongle will tell you this.

If an oxygen sensor fails, it will usually throw some variety of code on just about anything, to include pre-OBDII vehicles.  The problem is that they rarely fail, and only fail entirely at extreme miles.  The typical failure for a single wire sensor, especially, is that they get lazy and slow to switch.  The narrowband type (a 93 is almost certainly using a narrowband) doesn't really tell you how rich or lean you are, just that you are.  So, the engine computer enrichens the mixture until the sensor says rich, leans it out until it says lean, and just oscillates back and forth at cruise.  Sometimes this will get fed into a learning system that can compensate for a partially plugged injector or something when in open loop mode, but as the sensors get lazy, the crossings aren't as accurate, so you typically see a slow loss in fuel economy as the computer just isn't getting as rapid of data.  They're not "failed" - just no longer working as hoped.  But the computer can only detect a failed condition.  As long as they're switching, the older ones won't throw a code.

Newer, heated, wideband sensors (typically a 4 wire harness) give you more accurate information about the actual fuel/air mixture, and come up to temperature far more quickly (helps with cold start emissions).  I don't know as much about how they fail, not having owned many of those.

Quote
on an old car like this there is the possibility of end of life failure issues coming up on almost every part of the car, so it could be costly start swapping out parts. For myself, in the past, once I spend some money then the sunk cost fallacy kicks in and will lead to even more spending to make the already spent money 'less' wasted.

I generally disagree, having owned an awful lot of old vehicles that other people considered "end of life."  One I literally pulled from the junkyard, one I rescued from the junkyard in the "Junkyard offered $X, beat it and the car's yours" phone call variety.  Plus others that were hardly low miles.  I've run one vehicle to end of life (or, rather, various other people did - I tended to loan that one out for cheap), and when it went to the junkyard, it was done.  Just about everything that could fail was either failed or failing, and it didn't have a lot of systems to start with.  But outside serious rust (which can't be the case if a 93 is still rolling around), the only way you get a bunch of system failures is if you don't maintain the vehicle properly (in most cases).  Sure, you might need to put a new transmission in at some point (get a junkyard one from a wrecked vehicle, it'll probably be fine), and given enough miles on a high strung engine, you might want a junkyard motor, but I generally disagree with the "It's old, therefore maintaining it is pointless" approach to vehicle ownership I see depressingly often.

Also, if you're doing the work yourself, the cost is minimal.  If you have friends who you can trade labor (and maybe some garage space) with, it's even better.  I know this isn't a thing anymore in most areas, but... man was it nice.  If you have a group of friends who all work on older/cheap vehicles, cost to keep one running is quite low.

If I just replace the TPS or pay someone to replace the TPS, is that something that would likely fix this kind of issue?  Or is it more likely that a wire is loose or flakey somewhere and that problem needs to be addressed?

Go poke around the wiring harness for a while.  Oddball failures are rare.  If the whole harness is creaking and shedding insulation, you probably have a wiring issue.  Otherwise, the far more likely case is that the TPS is going bad.  They're usually variable resistors and they do wear out.

Quote
I'm inclined to think that the TPS itself is probably fine, and that it is a loose / bad / old / wire that is failing.  I'm also inclined to think that if I go to someone and say, "trouble code 41" that their inclination will be to replace the TPS rather than do what I would imagine to be fiddly work of checking wires and connections and voltages and stuff.  I'm also inclined to think that I'm not sure I could find a good mechanic who is willing to do the fiddly work rather than just replace-and-hope.

Why would you think the TPS is fine with that kind of miles?  Have you just replaced it?

If it were my car, I'd do the same thing a mechanic would.  Replace the TPS and clear the codes.  If you still have issues with a presumed-good TPS (I'd say known-good, but I've put bad parts on a car, new out of box, and so have most of my friends - quite the irritating issue), then hunt wiring, but unless you've got some very, very good reason to check the wiring, it's probably the TPS.

Do you have an old mechanical multimeter?  You can probably test it easily enough and see if it's smooth and within spec.  I'd use a digital one to see if it's within spec, and a mechanical one to make sure it moves smoothly, but... just replace it.

========

Bigger problem I see here is that you have a 27 year old vehicle and don't appear to be fully comfortable working on it yourself.  That will eat you alive.  You seriously need to be able to work on a car that old yourself, because nobody else is going to want to and will charge you the "Ugh, I don't want to do this..." rates.

secondcor521

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Re: Questions about replacing oxygen sensors on my car
« Reply #10 on: September 25, 2020, 10:14:15 PM »
@Syonyk, thanks much for the detailed reply with all the information and comments.  I really appreciate it.  I'll try to reply in order.

The symptom of this particular problem that I'm trying to fix is that the car just starts using fuel like crazy - it'll go from 23 mpg to maybe 6-8mpg seemingly overnight.  When the problem happened before, among other things I tried I removed the negative battery cable (to eliminate the codes) and that made the problem go away for about 8-9 months.  It came back a few days ago.

Two interesting key points:  The mpg loss is sudden and dramatic, and there are no other symptoms.  Except for a fuel gauge that I can almost see dropping by the mile, the car drives and handles fine.  So what you described about the O2 sensors gradually degrading doesn't seem to match my symptoms.

I'm happy to work on and maintain this car - it used to be my Dad's, in general it's a solid vehicle, and I know its history, so those are a few main points in favor of it.  What I think the other poster and I agree on, though, is not just shotgunning things but rather root cause and proceed logically.

(As an aside, my son has some friends who he's got an arrangement with.  They're a couple of older retired guys who like working on BMWs and they have every part and tool imaginable and then some.  He brings his car, buys the parts, serves as their helper, and they get to help him and teach him.  It's quite a nice arrangement.)

I did poke at the wiring leading away from the TPS sensor.  There were no obvious problems that I could see and it seemed in good shape, if 27 years old.

The two reasons I was guessing the wiring is that my Dad's Oldsmobiles tended to have electrical problems, and I could imagine more failure modes for wiring than I could for a sensor.  But admittedly I don't know the first thing about this stuff other than what I googled and Youtubed today, so I very well could be wrong.

I don't know the TPS is good.  It does appear to be the original, and I checked the service records and there is no record of it being replaced.

My plan is actually what you proposed - I've ordered a replacement TPS and will swap it out and clear the code and see what happens.

I don't have a multimeter but I have a friend who probably does and would be willing to lend it to me.

====

As far as your last comment, I'm comfortable on much of the basic stuff and am willing to learn more and do more within reason.  For me that means the price of the tools needed has to be within reason, that I can generally understand how to do the job (if there's a YouTube video showing how to do it and/or a Chilton manual description that's good enough), and something that isn't going to electrocute me or lose an appendage.

Over time I'm getting more comfortable as I learn to tackle more and more things on this car.  Fortunately, it has been very reliable and I've kept up on the general maintenance so stuff rarely goes wrong.  I also have a local mechanic who seems willing to work on my car even though it is old and doesn't seem to be charging "go away" prices.

Paper Chaser

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Re: Questions about replacing oxygen sensors on my car
« Reply #11 on: September 27, 2020, 06:01:19 AM »
Sensor failures are more common than wiring harness failures in my experience, unless there's been some physical damage to the harness. I'm not familiar with your specific model, but if you want to confirm if the TPS is bad and avoid the parts shotgun, I'd remove the harness @ the TPS and check the voltage with the key in the "on" position but without starting the engine. You'll have to determine what the appropriate voltage should be (6V is fairly common, but again it's vehicle specific).
Then I'd check the resistance of that circuit as well to see if it's abnormally high. If you can find a proper wiring diagram that will be helpful, and potentially worth a few bucks.

You'll need a multimeter for either test. I'd say it's a critical tool for anyone DIYing a vehicle new enough for electronic ignition/fuel injection, and they also come in handy for home electrical. If you can borrow one, that's great, but they're not all that expensive to buy either.


Syonyk

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Re: Questions about replacing oxygen sensors on my car
« Reply #12 on: September 27, 2020, 09:17:30 AM »
It's way easier to just use the multimeter on the TPS and see if it's out of spec and/or has jumps and glitches in the output.  If it's out of spec, the wiring is almost certainly fine.

Paper Chaser

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Re: Questions about replacing oxygen sensors on my car
« Reply #13 on: September 27, 2020, 11:31:07 AM »
It's way easier to just use the multimeter on the TPS and see if it's out of spec and/or has jumps and glitches in the output.  If it's out of spec, the wiring is almost certainly fine.

Maybe so, but that would only work for the resistance, and you'd have to know what the proper "spec" for resistance is for that specific sensor. You'd have to disconnect the sensor from the harness either way, so my thinking is that you might as well confirm that the harness is delivering the proper voltage to the sensor while you're there.

Syonyk

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Re: Questions about replacing oxygen sensors on my car
« Reply #14 on: September 27, 2020, 11:54:36 AM »
... yeah, you look up the TPS specs in the service manual, or check online for the ranges.  I'm pretty sure the various Haynes/Chiltons manuals had specs for things like that, but those went with the cars they were for when I sold them and I don't have any for the current fleet (haven't needed to go that far in yet).

There's no harm in testing the voltages at the plug, but actually ohming out the wiring harness is mostly pointless, because most glitches in a car are intermittent/vibration based.

secondcor521

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Re: Questions about replacing oxygen sensors on my car
« Reply #15 on: September 27, 2020, 12:22:14 PM »
The wiring diagram with the voltage ranges for my car is at the link earlier in this thread.  It doesn't seem to be that hard to check, but since (a) I've already ordered a replacement TPS sensor, (b) I can't figure out where the test harness is, (c) I don't have a multimeter, and (d) you people think it's more likely to be the sensor, I'm probably going to just replace the sensor.

(Yes, these things could be solved, but I'm not particularly motivated to solve them at this point.)

Also, an update - I reset things by removing the battery terminals for a few minutes, and that seemed to make the "bad mpg" problem go away.  I know it's not a permanent fix, but at least I won't be wasting fuel.  Replacement sensor due 10/5.

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Re: Questions about replacing oxygen sensors on my car
« Reply #16 on: September 27, 2020, 12:53:40 PM »
Please PM me your shipping address and I will send a multimeter your way.

zolotiyeruki

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Re: Questions about replacing oxygen sensors on my car
« Reply #17 on: September 30, 2020, 03:28:29 PM »
Sudden drops in gas mileage? Hmmm, my car (a '95 Corolla, so also a Toyota of the same vintage) did that a couple years ago.  In my case, it turns out I had accidentally disconnected the MAP sensor, so the ECU thought there was lots of air going into the manifold, so it ran really, really rich until I figured it out.  Basically, it had the same symptoms as a massive vacuum leak (I had unburned fuel coming out my tailpipe!).  Your comment about watching the fuel gauge drop visually reminded me of that evening...

So if the TPS doesn't fix it, I'd suggest you also take a few minutes to see if you have a vacuum leak.

secondcor521

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Re: Questions about replacing oxygen sensors on my car
« Reply #18 on: October 04, 2020, 09:18:20 PM »
OK, so I got the replacement sensor and replaced it today.  Everything seems to be fine.

But I was very surprised and a little disappointed by the online Chilton's manual that described how to replace the TPS sensor.  Their procedure was something like:

1.  Remove negative battery terminal.
2.  Depressurize fuel system.
3.  Drain radiator fluid.
4.  Remove throttle control and cruise control.
5.  Remove PCV tube.
6.  Remove air intake manifold hose thing.
7.  Remove throttle body bracket held in by 4 nuts.
8.  Remove three sensor connectors.
9.  Remove two throttle body bolts.
10.  Remove throttle body.
11.  Remove TPS sensor.

Which I did all of that (well, I skipped steps 2 and 3) and then got stuck on step 10.  So I looked at my engine a little closer and found out that I could do the following instead:

1.  Remove spark plug wire cover via six Allen wrench-style bolts.
2.  Remove TPS sensor.

Whoever edited that chapter of Chilton's is no longer a personal friend of mine, if they ever were.

Anyway, did steps 9 back to 1 (except 2 and 3) to put it all back together, did a test drive down to the nearby Fred Meyer gas station, and all is well.

Now to tackle the remainder of the 8 cubic yards of compost in my driveway :)

Thanks again to all for the comments, help, and advice.

Just Joe

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Re: Questions about replacing oxygen sensors on my car
« Reply #19 on: October 19, 2020, 09:37:12 AM »
Some of those manuals seem to be written by someone without access to the real car.