The Check Engine Light

[crokett]

in the Vibe has been off/on (mostly on) the last few weeks. I've triple-checked the gas cap. I had the code pulled. Advance says either O2 sensor or cat. converter. The car is running the same it always has, no issues, no loss of power, etc. So...

Assuming it were the cat, I should see a loss of power. Also, this would be bad because it means heat in the engine and long term heat is bad for the engine, right?

Since the engine is running the same (and the light is intermittent) I am thinking either O2 sensor or bad gas. I have been running the same gas I always have. They could have gotten a bad batch I guess though. My current plan is to run this tank almost dry and try a different brand. Is it possible there is some seal bad on the gas cap? Is there a way to test this?

[DUD]

David

In My van I had the 02 sensor trouble for awhile, couldn't tell if it was the upper, cheapest, or lower. I finally had it checked, of course it was the most expensive one. If it is the carylittic converter, it should be in warranty. Bill

[thrytis]

When my O2 sensor went, i could definitely tell something was wrong as the car ran rough. It was a turbo though, so i think it was more sensitive to such things than non-turbo engines. The only way i knew something was wrong with my cat. converter was the check engine light was on.

The cat. converter warranties often go a lot longer than the basic warranty. My previous car had a 20k warranty, but my converter was good through 80k miles (which was fortunate as it went bad at 79800 miles).

[LCHIEN]

The standard trouble codes are always Pxxx
I think gas cap has its own trouble code.
Many trouble codes come on and will indicate problems even with no apparent performance issues. But there may a pollution issue or other problem which may or may not harm your car long term but is harming the atmosphere.
The codes are persistent, most of them will stay on if the problem continues but till stay on also if the problem goes away, so there is a record of the problem if it is intermittent. It may take X miles, or N number of engine starts without the problem before the light goes out. N might be 50 or something.
What I do is I reset the code with my reader and see if it comes on again, eliminating a needless trip to the shop if there's a momentary glitch or something that clears up by itself.
if you replace a part (like the O2 sensor) then you should reset the computer's trouble code other wise you'll have to wait a number of days (50 starts might be a week or two) before you know the reults.

Aren't there several O2 sensors, as well?

[crokett]

My car has 2 O2 sensors, one before the cat and one after. Some cars have more. I think my van has 4. The computer compares the readings before + after and if it doesn't like the comparison will throw the code. I believe my car is it has to travel 60 miles before the code clears. In any case I don't notice any engine trouble. I will start with different gas and see what that does. I will have to check the warranty. The car is at 73k miles so is past the 72k mile warranty already.

[jziegler]

Dave, I think that the emissions warrenty is 8 years/ 80k miles. So, you probably can go to the dealer to get it checked/fixed. I know when I have my Maxima I had the cat replaced by the dealer free of charge at 70 some k miles. It was an unexpected surprise for me. Something that you should look into.

Jim

[crokett]

Dave, I think that the emissions warrenty is 8 years/ 80k miles. So, you probably can go to the dealer to get it checked/fixed. I know when I have my Maxima I had the cat replaced by the dealer free of charge at 70 some k miles. It was an unexpected surprise for me. Something that you should look into.

Jim

I will definitely get that checked out. I found this on a consumer buying site:

The federal government requires two other warranties. The Exhaust Emission Warranty covers corrosion-related parts for 2 years/24,000 miles, plus 8 years/80,000 miles on the catalytic converter and any on-board diagnostic device.

[Russianwolf]

One trick you can try is to reset the system. Simple. disconnect the battery for a couple hours. If the code comes back right away, then you definitely have a problem somewhere. Some codes are set to stay on until reset, others can reset themselves if the problem goes away.

As was said, the emissions warranty is 8 yr 80000 miles. I know this as our Saturn had to have an emissions control module replaced after a mouse shorted it out. Covered no charge at 75000 miles on a 5 year old car.

[rh111]

I usually don't advise this, but a bad cat conv. smells like rotten eggs from the exhaust. If you have a smell of that nature when the car is running, it is possible. The more relative guess would be the o2 sensor. Beware that once you put one in and it is running at efficiency, you might get a code for the other (most vehicles have 2).

[gjat]

I've had this happen more than once on my personal vehicle and on our company pickup trucks.

Look for a green cap, somewhat larger than an air-fill cap on a hose in the engine compartment. Check to see if it is tight, even a 1/4 turn would make a difference. You can take it off and put a bit of silicone lube (plumber's grease) on the inside threads and screw it back on. The light should turn off in a day or so.

I learned this trick from an honest mechanic who works on our company vehicles and my personal cars. It's a common problem that causes the light to come on and there are no other performance symptons. Dishones mechanics often take advantage of this to charge for new sensors or catalytic converters while only spending 10 seconds to "fix" your car.

[germdoc]

If your luck is like mine, it won't be a loose gas cap or simple solution. I had this problem with my Subaru Outback after about 1.5-2 years of use. It was a faulty oxygen sensor. Apparently that's a frequent problem with certain makes of car. Fortunately, it was covered under warrantly.

[Tom Slick]

In simple terms the O2 sensor before the cat is what the computer uses to adjust parameters for the engine. The O2 sensor after the cat is to ensure the cat is operating correctly. The rear O2 sensor being bad could show up as a bad cat or bad O2 sensor.

[cgallery]

One trick you can try is to reset the system. Simple. disconnect the battery for a couple hours. If the code comes back right away, then you definitely have a problem somewhere. Some codes are set to stay on until reset, others can reset themselves if the problem goes away.

As was said, the emissions warranty is 8 yr 80000 miles. I know this as our Saturn had to have an emissions control module replaced after a mouse shorted it out. Covered no charge at 75000 miles on a 5 year old car.

This may not work on all cars. I've had some people bring me car computers to work on (installing performance chips, replacing caps, etc.), and I've noticed that some of them have NVRAM on them.

[mpc]

The original poster mentioned he'd "checked the gas cap" which is the most common source of the check-engine light... folks just don't tighten them enough. Some though have a screw or two holding them together and when that gets loose the cap won't seal properly so look for that. Or a rubber gasket that's dried out/cracked.

Subarus seem to pick up cat converter worn-out errors or bad 2nd oxygen sensors. As Tom said, that after-cat sensor is just to monitor the cat converter's efficiency. If you can clear the codes... and then pay attention to when the code re-appears you might be able to narrow it down a bit: cat converters work when they are good and hot. If the code shows up during light around-town driving then it's likely the cat converter since that type of driving doesn't create high exhaust temps to keep the cats as hot - so marginal cat converters flunk the "cold test." If the code shows up when accelerating hard, carrying a heavy load at highway speeds, or going uphill - that's making lots of exhaust temps and giving the cat a little extra to work with so I'd lean towards an O2 sensor that isn't up to snuff or a problem with the engine's fuel control leading to an over-rich mixture. Flakey connections to the O2 sensor "heater" also cause these problems on sensors with heaters.

If your car is still under the emissions warrantee though take advantage of it. A new/free cat converter is a total gloat!

gjat - what's this green cap you described? And what is the hose connected to? My cars are all too old for this OBD-II stuff and I don't recall anything like that on the parent's Subie...

mpc

[crokett]

I switched brands of gas last week and so far that seems to have done it. Up until then the light was only off for 24 hrs or so before it came back on. Now it has been off since Tuesday.