After having lunch with my kids locally, the "CHECK ENGINE" light came on in my 2001 Subaru Impreza 2.5 RS. That ruined my plan to drive out to a far away Best Buy. So we drove home. While driving home, I can feel my car idling roughly.
When I got home, I plugged in the Actron CP9175 OBD II AutoScanner to pull the engine code:
I learned a trick from my mechanic once. When you get a P0301 code, you can swap the cylinder 1 and cylinder 3 spark plug wires, reset the code, and drive around a bit. If the code come back as cylinder 3 misfire, then it's the spark plug wires. If the cylinder doesn't change, then it's something else.
So that was my first troubleshooting step. I pulled off the cylinder 1 spark plug wire from the distributor. And there is this weird metal piece on the distributor connection. I pulled off the cylinder 3 spark plug wire and it didn't leave this weird metal piece on the distributor. Apparently, the connector inside the spark plug wire had came off. Hoping that was the problem that caused the code, I proceeded to AutoZone for some new spark plug wires.
After installing the four new spark plug wires, I took the car out for a spin. The "CHECK ENGINE" light was still on. I wasn't sure if the P0301 code will clear by itself. And the engine was still running rough. I got a little worried that maybe the spark plugs weren't the problem. But I used the Actron CP9175 OBD II AutoScanner to reset the ECU anyway, which said that I had to shut down the engine first.
After resetting the ECU, I started the car up. Surprisingly, the engine ran extremely smooth compared to earlier. I drove the car around locally and on the freeway, it felt fine. I parked and plugged in the scanner and there was no code. I drive fifty miles more and the engine was still fine. So the spark plug wires were the culprit.
In hindsight, I think when the ECU detects the P0301 code, it immediately goes into limp mode to avoid damaging the engine. Even after the spark plug wires are changed, the ECU still assumes that the misfire exist and stays in limp mode. It isn't until I reset the ECU that it comes out of limp mode.
This past month, the car felt sluggish. It was difficult to get it up to speed and stay there. After the spark plug wires were changed, the car was as peppy as ever. I was able to get it up to speed and stay there. I didn't realize brand new spark plug wires can make that much difference.
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