The dreaded "check engine" light came on yesterday.

[nitroman58]

On my son's 2001 Forester L, the check engine light came on after he hit a bump in a road. The car is running fine otherwise. He took it to a local Auto zone where he was told that it was either the oxygen sensor or the catalytic converter. The car has about 85,000 miles at present.

Other than being certain that the gas cap is tight, are there things I should check? My mechanic will look at it on Friday. I sure hope it is not the catalytic converter! I believe that is a big ticket item.

[1 Lucky Texan]

probably the P0420 code. DO NOT replace the cat conv based on some text from a scanner, many things can cause various codes and it may not be anything more complex than an exhaust leak or disconnected wiring harness.

 

a good mechanic can perform tests to determine if a sensor or cat converter is bad. certainly converters DO go bad, but sometimes a much less expensive repair is all that is needed. proceed carefully - even getting a second/third opinion before dropping mega $$$$$.

[NV Zeno]

My Outback has done that a couple of times over the past couple of years.  Same "diagnosis" from scan tool.  After a few days, (maybe less) it just went out. 

 

Haven't had that happen in almost a year.

 

Just my 2 bucks

[jp98]

Did he have them clear the code to see if it comes back? 

[Fairtax4me]

How big of a bump was this?

 

It would be helpful to know the actual code number, but based on the comments from autozone It sounds like its a P0420 code.

There are tons of reasons the 420 code can be set. Most of the time it is NOT because the catalytic converters are bad.

 

Now depending on how big that bump was, the cats could be physically damaged, which could set the code after driving for a few days.

If the light came on immediately when the bump occurred that's probably just coincidence.

[mikaleda]

I'm very doubtful your cat is the problem, especially since its running fine.

I would check for leaks in the fuel filler neck, its common for them to rust out and cause an air leak similar to a loose gas cap. You'll have to pull the plastic cover off to inspect it properly.

[Rooster2]

I agree with others, don't replace the cat converter. These days they can last the life time of the vehicle. I doubt that the AZ store would remove the check engine code. My experience is that they simply will tell you what the code represents, you have to ask them what code number appeared.

 

In my similar experience, it may have been a loose gas cap. That will set off the check engine light.

 

To get rid of the code, pull off one of your battery cables, wait a few minutes, then reinstall. The check engine light should be off. Make sure the gas cap is on tight. Maybe the tighten cap will solve the problem. If not, get code read again, this time, ask what code number appears. Post what that code number is..........that will help in future diagnostics.

[keith3267]

Please go back to AutoZone if the CEL is still on and have them read the code and this time write down the code yourself or have them input the code into their computer and give you the cash register printout for the code.  The code will be at the top of the printout.  Right now, everyone in just guessing.

 

If AutoZone erased the code and it has not come back, or if it went out on its own, then do not worry about it.

Edited March 11, 2015 by keith3267

[nitroman58]

Wow, thanks. The car is going to my local mechanic tonight. He is pretty good with not charging for things that do not need to be done.

One thing is, the gas cap is an aftermarket model. It seems to go on tight, but it does not give that clicking noise when turning it clockwise.

[idosubaru]

P0420

 

an exhaust leak causes the P0420 code and could be jarred by a large pothole.

almost anything can cause a P0420 code.

 

a $5 extender on the rear O2 sensor is a cheap work around for what is a benign issue by itself. 

[plogerjb]

Check the operation of the rear o2, front 02, then cat, in that order. Can be done with any datalogging scanner or by any decent mechanic. Any physical damage should be corrected first, of course.

[Rooster2]

Wow, thanks. The car is going to my local mechanic tonight. He is pretty good with not charging for things that do not need to be done.

One thing is, the gas cap is an aftermarket model. It seems to go on tight, but it does not give that clicking noise when turning it clockwise.

I have two Subies, both with after market gas caps. Both don't click when tight like the OEM cap. Makes no difference, if the cap is tight, and the gasket on the cap is good, then it will work just fine, won't throw a CEL.

[idosubaru]

i hear three things all the time that i've never seen in person and rarely happen to Subarus:

"fuel filter clogged"
"catalytic converter might be clogged"

"check your gas cap"

 

i'm sure it happens but the discrepancy between how many times it's suggested and how often it happens is enormous.

i want to go intentionally loosen mine just to see if the CEL will come on on my drive home. LOL

[forester2002s]

A loose gas cap will set code P0440 'Evaporative Emission Control System Malfunction'.

[nitroman58]

My mechanic checked the code today and it was P0420.  He reset the code as well.  I drove the car today, and I thought that is ran pretty darn good.  I will just see if it comes back on for now.

[plogerjb]

Heads up, resetting the code doesn't really do anything. It resets your monitors and turns the light off, sure. However, those same monitors will come back up in the same amount of time that the light would have gone off on its own if the problem was corrected or intermittent. Check engine lights clear themselves if the issue no longer exists. Six to one, half dozen to another I suppose, just saying it would be a waste of your time to go out of your way to reset the light. A p0420 isn't going to change the operation of the engine whatsoever, as the ECU is going to do its best to continue checking for catalytic converter operation. 

[brus brother]

Had intermittent 0420 for the past 6 years on my 2005 OB. Changed 2 passenger side O2 sensors (my car has 5 in all) but refuse to change cat. Car runs great. Once in a while, the light goes out for a while. I bought a cheap scanner for about $18 on ebay that erases the codes and there is a sequence to reset the systems so I can pass emissions every 2 years. Don't mean to be a polluter. I just think these model years are just set too sensitive on the parameters which is why the light will go out on its own now and then. Right now it's off. Had a piece of black tape in front of the lights for a while but not wanting to attract attention, I removed it for emissions testing and never replaced it.

Edited March 14, 2015 by brus brother

[ThosL]

I had one of these; actually a local Advance Auto had their ability to do check engine scans pulled after they were resetting the computer for customers; but the Auto Zone is still doing it.  Info I got was that the guaranteed to pass product will help clean it out or Sea Foam; but like others said here a real diagnostic is indicated first; unfortunately I am loath to pay $100 for one of those around here so will try to apply what is said here.

[crazyhorse001]

My 2cents... 0420 code is a generic code set when the last o2 sensor sees a funky reading. The best thing you can do is, double check the connector, and condition of that sensor. If everything looks ok, and the light doesnt come back on, leave it be. On the other hand, if it DOES come back quickly, that bump may have jarred something loose inside the sensor.

 

I reccomend anyone with a '95, or newer car buy a cheap code scanner. $69 is way cheaper than taking it to a mechanic, and will give you the first line of defense to keep your car running its best. You dont have to be mechanically inclined to use it. If you can plug in a lamp, you can use a scanner.

[brus brother]

I use the Autel MS 300 around $16 on ebay shipped reads and erases codes. You can get cheaper units that require computer interface but this lil piggie plugs and plays in an instant reads and erases and off I go to the emissions testing station I go once the systems are reset.

Edited April 17, 2016 by brus brother

[idosubaru]

My mechanic checked the code today and it was P0420. He reset the code as well. I drove the car today, and I thought that is ran pretty darn good. I will just see if it comes back on for now.

got it.

P0420

it has to run good. the ECU doesn't even use the sensor for the P0420 code to run the engine. you can literally remove it, cut the wires to it, and your car will drive exactly the same.

unless there's some additional symptom/issue and this is just a phantom code, but that's rare and you haven't described anthing else - besides a bump which is unclear at the moment.

you might have an outlier but in general they'll come back at some point in the future.

 

converter replacement is usually not needed but often the only consistent approach for repeatable diagnosis and repair.

general mecanics rarely specialize in familiarizing themselves with all makes, models, and engines variants, but here's what a thorough diagnosis begins to look like - though it's very possible not a mechanic in your town even does this:

 

http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diagnosis.html

 

Edited April 17, 2016 by grossgary