Poor performance after clearing trouble codes

rbaer · 06-27-2011, 07:42 PM

Does anyone know if you can damage an ECM by clearing diagnostic codes while the car is running? I was using a generic scanner to read out trouble codes and immediately after clearing codes, the car sputtered, and now it accelerates very poorly at anything other than a slight throttle increase.

This is a 1999 boxster 1999 2.5. The codes were for both precat O2 sensors being out of range. When I first saw the codes a few days earlier, I decided to first replace the spark plugs. That went well and the car ran beautifully for a few days until I cleared the codes.

I've considered trying to swap the ECM on it, but I'm hesitant to buy one because I can't tell if mine is in fact bad. I don't know if there is any additional programming needed or if I could just drop in a used similar ECM and see if that fixes the issue. Any suggestions are appreciated.

Dale_K · 06-28-2011, 06:10 AM

I think clearing the codes also clears the long term fuel trim tables. So right after you clear the codes the car can run poorly but gradually get better as the ECM adapts the fuel mixture and makes adjustments based on what it sees from the O2 sensors. It's basically the same thing that would happen if you disconected the battery.

rbaer · 06-29-2011, 09:58 AM

I've heard that clearing the codes can clear the fuel trim tables as well, but my experience was that when I did it with the code scanner with my engine off, it had no effect other than to turn off the check engine light. When I did it while the car was running, something different happened and maybe it did just clear all the fuel trim tables. Regardless of exactly what happened, the car ran horrible like it was in limp home mode. I was mainly concerned that I may have corrupted the computer or fried something.

I did a lot of troubleshooting since this happened and had driven the car a lot with little or no improvement. So last night I pulled the battery for about an hour and pulled and tested all the 02 sensors. They all seemed fine, but I switched their locations to see if I would eventually get a trouble code on the same 02 sensors in new locations. After doing that, the test drive went very well. It wasn't accelerating as strong as it should, but it does seem to be relearning the fuel trim values now as it is running better on successive trips.

No new codes yet, and no explanation for why it tripped the check engine light in the first place. I'm wondering if it was just a tank of bad gas.