Car emissions code

[Travis]

Been getting the check engine light on our 2008 Prius the last month. We've taken it in 3 times and each time the mechanic says it's the emissions sensor. We've tightened the gas cap, filled it at least once, and driven it around for about a month and the light still comes on. The shop's suggestion at this point is to replace the gas cap. That's a cheap fix if that's the case, but the cap was replaced three years ago after a failed emissions test so I'm concerned that there might be something else going on. Thoughts?

[BudgetSlasher]

Without the actual code, usually in the format of P0### (for emissions I believe the first # will be a 4), anything would be a shot in the dark or at making assumptions based on the mechanic. But here goes mine.

If the code is P0455 (Large EVAP leak) it could very well be the gas cap, in fact google says gas cap issues (including the owner not sufficiently tightening the cap) are the most common cause of the code. I only know that from past troubles, google says there is a small EVAP leak that is likely something else.

There are also a good number of codes that have to do with emissions overall, but in no way would point to a gas cap.

For me if the code was P0455, I would start by picking up a new gas cap at an auto parts store for ~$15 and installing it myself and making sure it is properly tightened every time it is used. If the light is on most auto part stores will read the code for free and/or clear it. Google says there are other potential causes of the codes, but it looks like the cost of the parts range from ~70 to ~360 and then there is labor.

[Travis]

DW took it in, so I'll have to call the mechanic and see if they kept a record of the actual code. Gas cap ordered and will be here in a couple weeks. We'll see if that fixes it.

[iluvzbeach]

I would recommend a Toyota gas cap for your specific model vs. a generic gas cap. We had this issue on a Honda and it was simply that the gas cap hadn’t been tightened properly & even then it took a bit of time for the light to turn off.

[Greystache]

FWI, you can usually go to your local auto parts store and use their OBD reader for no charge. I bought my own. You can get a basic unit for around $50. It's a good tool to have.

[Syonyk]

Buy an OBD-II reader of some form or another.  They're dirt cheap, and those cheap ones will work with just about anything, though make sure it's compatible with your phone - I've got one that works with Android but not iOS, so I just keep an old Android phone around for my OBD-II chatter (it literally lives in the car).

Then Google the exact code you get on a vehicle specific forum, and try the stuff mentioned there.

[JLee]

Something like this will do the trick, and doesn't require a phone - https://www.amazon.com/gp/product/B001LHVOVK/

I like the simplicity of units like that (no bluetooth or wifi pairing/etc, it just works).

[Dave1442397]

Yes, buy a code reader. This one was recommended by a mechanic, so I bought it - https://www.amazon.com/gp/product/B071FSNL73/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

It told me exactly why I had a check engine light on (low coolant), and it's nice to have it to hand.

[Travis]

I would recommend a Toyota gas cap for your specific model vs. a generic gas cap. We had this issue on a Honda and it was simply that the gas cap hadn’t been tightened properly & even then it took a bit of time for the light to turn off.

We ordered it from in-laws' local dealership and it's in the mail.

[Car Jack]

Mechanic said "emissions sensor".  Uh, ok, what sensor?  What code?  What did he say it would cost to fix?

My no info guesses based on info we don't have:

If he said it'll cost $150 to fix, that means it's an oxygen sensor.  These are easy to replace, but depending on the cost, if you replace a completely good one, you'll spend more on sensors than paying him the $150.

If he said it'll cost $1500, then you likely have a P0420, cat efficiency too low code which can be the cat at worst but can be a buncha other things as simple as a leak in the exhaust.

In short, this is a particular problem that you should bring it to a competent mechanic who is going to actually diagnose the problem, find the reason, then have the solution to the dollar. 

I am very familiar with many car problems.  But I don't have the high end (thousands of dollars) OBD scanners to look at output of the ox sensors on the screen and such.  For those problems, I bring it to a "real" mechanic and let them diagnose and fix the problem.