Troubleshoot my Honda's Check Engine light.
July 26, 2008 8:09 PM   Subscribe

My wife's '98 Honda Accord's engine light is on. Help me troubleshoot it to cross-check the mechanic's diagnosis.

Mechanic A said the ECU (Engine Control Unit) spat out a code to replace the oxygen sensors, so they did (with non-Honda parts). The engine light remained on and gave the same code again, so Mechanic A concluded that the ECU is busted. We took it to a Honda dealer for a diagnostics check, and they received the same code and said it needs Honda-brand oxygen sensors, but that the ECU is fine. Back at Mechanic A, they put in Honda oxygen sensors for free, and the Check Engine light and the same code remains.

-I have heard that a car's computer/ECU rarely goes out. But does this sound like the culprit?
-I'm skeptical that they've reset it properly. I've done a bit of research online and have tried disconnecting the battery for 20 minutes, but the light STILL remains. Any other resetting ideas?
-Mechanic A quoted me a USED ECU incl. re-programming it for about $500, whereas a new one from Honda is $1400. Is there a high risk for going with a used ECU?
posted by blastrid to Travel & Transportation (17 answers total)
The ECU is generally the last item on any Check Engine Light diagnostic procedure. They rarely fail. However, the fact that they cannot reset the light at all, for any length of time, suggests an ECU problem. 20 minutes with a disconnected battery should almost always reset the light, at least temporarily.

Used ECU's can be a crapshoot but Hondas are quite accommodating to changing in/out these kinds of parts. I wonder what Mechanic A's qualifications are, however? I would be more inclined to stick with the dealer as long as you intend on keeping the vehicle.

As for oxygen sensors, it is not a good idea to replace them with non-OE parts, as aftermarket parts (which can be used on many different models of vehicles) can have different resistances and might read incorrectly, especially depending on who is doing the splicing if the wiring harnesses don't match. FYI.
posted by jeffrygardner at 8:31 PM on July 26, 2008

Has anyone said what will happen (aside from the engine light staying on) if you don't replace the ECU? For $500-$1400 on a 10 year old car, I'd be tempted to just ignore the light and keep driving it, as long as no one was threatening me with dire consequences for doing so.

I also had a problem with a non-OEM oxygen sensor causing trouble on an old car I had; changing it to the much more expensive one from the dealer fixed that problem. It was an expensive learning moment for me, because the first O2 sensor wasn't returnable, so I ended up paying for both, thanks to trying to save money.
posted by Forktine at 9:04 PM on July 26, 2008

@forktine: A check engine light (in Arizona, at least) will cause an immediate fail of yearly emissions. And blastrid's fuel economy is prolly less than it should be.
posted by jeffrygardner at 9:13 PM on July 26, 2008

There are some fairly inexpensive gadgets that you can plug into the diagnostic port (OBD-II) and read the codes yourself - some have a USB interface that can download the data into your computer.

I don't have any first hand experience with these, but wikipedia has some more info.
posted by kenliu at 9:24 PM on July 26, 2008

Resetting idea: If you go to any Advance Auto (do they have those in AZ?) they will lend you an OBD-II reader and you can read the codes and reset them from there. If it's a real problem, the CEL should come back on later, after driving the car for a while. If you don't have Advance, maybe another car parts store will do the same thing.

ECU idea: Do you know a friend with the same model Accord, with the same engine and transmission? How good of a friend are they? It doesn't take long to swap an ECU, and if you tell your friend you will pay for any damage to their ECU that might be caused accidentally, you may convince them to let you swap ECU's for a few minutes to see if that light goes out. Depending on how likely your friend is to blame any subsequent (unrelated) problems with their car on you, this may not be worth attempting. I'd do it for a friend of mine in a heartbeat though...
posted by no1hatchling at 10:46 PM on July 26, 2008

It could also be some fault in the ECU programming itself. Our '01 Maxima has an eternal CEL and code that indicates that the catalytic converter is bad. After much investigation, we discovered that this is a false code due to some bad programming in the ECU. It's a fairly common issue with this particular year Maxima and not correctable.

So we drive with the CEL on and occasionally have the codes read to make sure something else hasn't popped-up.
posted by Thorzdad at 4:57 AM on July 27, 2008

There are some fairly inexpensive gadgets that you can plug into the diagnostic port

On my Accord [96?] you could actually put a paperclip in some little port you could reach under the floor carpeting on the passenger side and read the codes yourself, check your manual or a Chilton guide. My Honda had a very similar problem -- it's my ex-car now so I'm sorry if I'm not remembering this all properly -- and it turned out there was some sort of manufacturing flaw resulting in an extended warranty. There's a little bit of information here and I think it covers your car and should give you ideas of where to go for more info.

That said, your dealer should know this so it's quite possible that this has nothing to do with your problem, but when my check engine light came on with an O2 problem and I got it fixed at my local mechanic's and later got the letter from Honda that this was a covered repair, I actually got Honda to pay for the repair retroactively which sort of blew my mind.
posted by jessamyn at 6:02 AM on July 27, 2008

Another vote for the light being the issue, rather than the ECU. It is unlikely that the ECU is faulty with no other signs of an issue (I am assuming that fuel economy and power seem the same? No coughing, spluttering or oddness?).

If at all possible, get another ECU swapped in and try and reset the light (as mentioned).

If not, it doesn't sound like you went back to teh dealer after Mech A refitted the OEM sensors. Perhaps do that, and ask them if there is some issue that will cause the light to remain on.

Again, driving with the light on is perfectly fine. I'm surprised that a check engine light is an automatic fail for emissions - it assumes a government total faith in the engine diagnostics. By the same logic, if the light is NOT on, they should assume the emissions are fine for a given model of car. Being as they (rightly) don't, even acknowledging the light is nonsensical. They are still a long way from 100% reliable. It shouldn't matter at all, as the stuff coming out of the exhaust is the issue, not how bright the dash area is. I wonder if you can either get the emissions done by mechanic A or the dealer that know the light won't go out if it is an issue. If you aren't in an area that matters about the light, I'd just keep driving it and checking occasionally, as mentioned.
posted by Brockles at 6:59 AM on July 27, 2008

ECU going bad happens very rarely, but it does happen. A friend's Civic of the same vintage had to have it replaced.
posted by electroboy at 8:29 AM on July 27, 2008

I would suspect a problem in the wiring or connector terminals between the ECM and the O2 sensor.
posted by rfs at 10:10 AM on July 27, 2008 [1 favorite]

mine has been on for 10 years: crossing my fingers!
posted by dougiedd at 2:37 PM on July 27, 2008

Try and find the factory service manual. The Nissan one has VERY SPECIFIC guidelines on how the O2 sensor operates. It is something like:

"0-2 minutes, puts out 5 volts, after warmup produces 2.5 volts" or something like that. It also has cases for when it puts out different voltages, and ways to test the unit.

You should be able to find out exactly what pins on the ECU are being used for the o2 sensor, and measure them with a volt meter. I suspect wiring or terminals such as rfs suggests.
posted by SirStan at 2:40 PM on July 27, 2008

When was the last time you had dealer maintenance done on the car? My Accord's Maintenance Required light was on for four years because I wasn't able to take it in for regular maintenance every 30,000 miles. Once they checked, they switched off the light.
posted by mynameismandab at 6:43 PM on July 27, 2008

Oh wait sorry, I didn't read the rest of it...Scratch that.
posted by mynameismandab at 6:44 PM on July 27, 2008

My Certified 2004 Civic (bought in 2007 with 36,000 miles) had its ECU die at 37,500 and it did the same thing you are describing. My friend's 2002 Civic died at 19,000.

Everyone around me and I myself have had nothing but scary problems with our Hondas. I am beginning to doubt the wisdom of my purchase. Although I've driven a standard my whole life, my Civic's clutch and flywheel were destroyed by something called a faulty input thust(er?) bearing at 59,000 miles.
posted by vkxmai at 8:39 PM on July 27, 2008

Response by poster: Thanks, all, for your input.
A few things:
We would gladly keep driving it and ignore the light, but the car's registration is expired and emissions fails because of that damned thing. I've since tried the "maintenance req'd" reset by pushing the tripometer reset button while turning the car half-on, after having disconnected the battery for a half hour. it re-illuminated.

Everything else in the car seems to be running smoothly (no power or stutter issues). I unfortunately don't have a good friend with the same make/model.

I tried to get Mechanic A to swap an ECU in there for free to see if that would finally fix the problem--so i didn't have to shell out $500 to fix a perfectly good unit--but that can't be done. It has to be reprogrammed to my VIN (read: purchased). Mechanic A has been a reliable shop in our city for 22 years, but we've been going consistently for the last 6. I am reluctant to go to the dealer because I cannot afford $1400 for a brand new ECU.

I know which code came out (o2 sensors), as both Mechanic A and the dealer showed the same one to me on their print-out.

From a general troubleshooting point of view, it really points to the ECU, imo.

--both my wife and I have Hondas (my 3rd), as do my brother and his wife. We are all extremely pleased at their general dexterity and efficiency. Except for weird annoying issues like this. Dammed Check Engine Light!

For the AskMeFi record, once we get the ECU replaced (I'll probably bite the bullet and do that this or next week), I will post the results here.
posted by blastrid at 11:29 PM on July 27, 2008

If I were you I'd go to one of the many honda forums out there and research the code. You're probably not the first person to have this problem.

My understanding of CELs is that checking the CEL source will give you the sensor that gave a bad reading, which isn't necessarily the sensor that is bad. Your o2 sensor could be reading something that isn't correct, so there could be an issue with something that precedes your o2 sensor in your car's intake/exhaust system.
posted by PFL at 7:10 AM on July 28, 2008

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