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Fried PCM after O2 Sensor Replacement

My 99’ Acura 3.0 Cl (6-cyl) was giving me code P0141 (downstream o2 sensor heater circuit) so I bought a new OEM Denso 234-4601 from Amazon and took it to a local reputable shop to do the replacement. When he went in to reset the code, the car started running very rough and gave ~10 error codes including one in the transmission. After the shop was unable to diagnose what was wrong, we towed it to the dealer where they said the PCM was fried. The local shop said they would cover some of the cost of the repair but wants to buy and install a used part. The dealer, though, wont go along. I’m not quite sure what to do from here.

My questions:

  1. Can a bad O2 sensor cause the PCM to short?
  2. If not, how could the shop have shorted the PCM while installing an O2 sensor (it was a rainy day and they did the job outside. also, key was in ignition)?
  3. Could anything else be wrong with the car in addition to the fried PCM?

Im just trying to figure out where the responsibility lies. Any help would be greatly appreciated.

Update (01/17/14):
The dealer confirmed that the new O2 sensor was good. That leaves the responsibility with the shop. The shop offered to do a used PCM so I asked them to contribute what their cost would have been and will pay the dealer the difference to get it done correctly.


Was the battery disconnected while the sensor was being replaced?
It’s possible (a wild guess) the PCM was damaged with the battery still connected.

I believe that the battery was still connected. Is it standard practice to disconnect it when doing an 02 sensor replacement?

As for responsibility, if the shop has already agreed to cover some of the cost, it’s pretty clear they know they did something wrong. Someone shorted something out by accident. It can’t get any worse than frying the PCM, so I wouldn’t worry too much what else might be wrong. Denso sensors are good so I doubt the part was at fault.

The dealer of course wants to sell you a new part and doesn’t want to be blamed if they install a used part that later fails. You may have to take it back to your local shop amd let them install a used PCM.

Out of all the O2 sensors I’ve replaced not once did I disconnect the battery. There’s no reason to.

As long as the ignition switch is off there’s no power to the O2 sensor. The O2 sensor produces it’s own voltage signal to the computer. And this is only 0-1 VDC. However, the computer grounds the heater circuit for the O2 sensor. So if the computer fried after installing the O2 sensor, there may have been a problem with the heater in the O2 sensor itself.

So a defective heater in the O2 sensor is the only way to fry the computer.


Or they did something like let the battery go flat, then jumped it incorrectly.

Personally, I don’t think the new sensor caused the pcm damage

I believe the shop may indeed have buggered up the car, and isn’t going to say exactly what mistake they made

It might not hurt if the OP would clarify the term “immediately”. Some use that term to mean then and there, hours, days, or even weeks later.

It’s pretty dicey territory seeing as how a shop waded into a repair with a customer provided diagnosis and customer supplied part.

The dealer can’t be blamed for not wanting to get involved with a used PCM either; and that’s assuming for the sake of discussion that the PCM is really “fried” as it’s referred to.

Batteries do not need to be disconnected to replace a sensor, or much of anything else for that matter. In this day and age disconnecting a battery can cause more trouble than leaving it connected.

Red flag #1 is a shop doing repairs to a car outside in the rain.
Red flag #2 is a shop installing an outside (customer provided) part like an O2 sensor without any indication that said part is bad. Fault code P0141 could be caused by any number of things.
Red flag #3 is a shop that is unable to diagnose a check engine light and rough running condition on a relatively simple car.
Red flag #4 is the shop agreeing to pay part of the cost. Either they know something happened during the installation process or they’re letting themselves get beat up under the guise of customer satisfaction.

Someone–I assume the dealer since that’s where the car is–is going to have to do some testing. Is the heater element in the new sensor faulty? Was the old sensor? Is the new sensor faulty in some way? (I’ve seen brand new factory parts miswired). Is there any evidence of wiring shorting out somewhere? Has the dealer documented how it came to the conclusion that the PCM has failed?

Thank you for all the suggestions. To clear up some questions, the problems started as soon as he went in to reset the original code so it definitely fried during installation. Also, I asked the dealer to check the new part and it checked out. Finally, two different shops had confirmed that I needed a new o2 sensor before I bought the part and the shop that did the work confirmed the diagnosis and checked to make sure the new part was the correct one.

At this point it seems like the shop did something during installation and its not the fault of the new part. Although the shop is offering to install a used PCM, I may just ask for a check and pay the difference to get the dealer to do the job.

I wouldn’t worry very much if a used ECU was used for a replacement as long as it works okay after it is installed. They are usually pretty reliable. A new one from the dealer would most likely be big bucks and a used one from a salvage yard might be under 100 dollars. I think it is nice of the shop to offer to help with the cost of a replacement. I’m not sure how they could of caused damage to the old one. It would be nice to know what is wrong with it. It could be possible that there may really be nothing wrong with the ECU if the shops didn’t check things correctly and perhaps there is something wrong within the wiring. It would help to know what the error codes are. If they are related to circuit error problems that would be a clue there may be a power supply problem somewhere external to the ECU. Hopefully the shop has made sure that isn’t the case.

Working outside in the rain

I have personally seen a few problems caused when water got in electronic components . . .

The reason some car repair manuals and experts suggest the battery ground be disconnected during the repair or part install process is that there are hot (powered-up) wires in the engine compartment even with the key out of the ignition. The thick wire going to the starter motor for example. A wrench could hit an exposed terminal and cause a high current short, no telling what damage doing this would cause, it would depend what the wrench was touching, but damaging the PCM would be a possibility. I always disconnect the battery ground for this reason whenever I work on either of my cars.

Whether this happened in your case, who knows? But replacing an O2 sensor wouldn’t usually damage the PCM unless something like that did happened. More likely though, it had nothing to do w/replacing the O2 sensor, it was the attempt to reset the CEL that damaged the PCM. Some spark or other problem when the connection of the PCM port was made to the scan tool maybe. Or the scan tool was defective or something incompatible about it. On some cars the scan tool can only be safely connected to the PCM connector when the car is in a certain mode.

There’s one other possibility. Maybe the PCM remains fine and in perfect operating condition, and that the PCM had learned how to adjust the engine operating parameters to compensate for the problem of the old O2 sensor. When the new O2 sensor got installed, these parameters no longer worked. Maybe it just needed some driving time to relearn the new parameters. Or the new O2 sensor is no good.

The car was running so rough and rich after the replacement that the engine would cut out after ~10 seconds so I am not sure that driving it would have been a good idea. There was also a transmission code with D4 blinking on the console.

At this point, I am leaning to an explanation where the combination of a wet part (either code reader or the new O2 sensor) and power to the system caused something to short. It is at the dealer now and they diagnosed it as a bad PCM. I got the shop to pay me what their cost would have been to do a salvage part and will pay the difference to the dealer to get it done right as I don’t have a lot of confidence in the shop after this ordeal.

Keep us informed as to the results. The thought keeps coming to mind of what if the replacement PCM does not cure the problem…

@CL85 … hopefully your plan will fix it, but fyi there are companies that will test and repair defective modules like PCMs. “Module Master” is one of them, and there are others, Google can find them for you. I’ve never had to use any of them or ever needed a module repaired but just in case, that could be a resource for you. Another good thing about those module repair places is they have people there w/a lot of expertise on the modules and often will help you to diagnose if it even is the module or not that is causing the problem.

I got my car back from the dealer today and it is working perfectly after changing the PCM. They left in the O2 sensor that the first shop had installed. Thus the part was good, leaving the responsibility with the shop. They agreed to pay me what their cost would have been to do a PCM replacement and I contributed the difference.

In talking to the shop, the only thing they could think of was that the mechanic cut the cord for the O2 sensor while it was connected to the car. If the computer had power to it, this could have shorted it. We’re still a bit puzzled but that’s the best we could come up with.

Thanks for everyones help.


Thanks for the update

I still think the problem occurred because they worked on your car, outside in the rain