JonCAGS300

My battery light came on. I had the system tested: bad battery and alternator. First I replaced the battery, and the battery light stayed on. Now I just replaced the alternator. I had the battery disconnected during the install. When I started the engine for the first time, the battery light stayed off, but the check engine light came on. It wasn't on before that. I checked the code, it's 21: Main O2S & Heater Signal. I have tried disconnecting the battery again and pulling the EFI fuse, but the code is still thrown. How can code 21 be related to replacing the alternator?

Kruso

Welcome to the club... countless members here have had Code 21 thrown and many have replaced (both in some situations) O2 sensors and still never were able to resolve the issue. The problem is that the diagnostics codes are a bit misleading. If testing is done in a dark area, Code 21 may set even though system is normal. Shine a light at solar sensor and recheck codes.

Kruso

JonCAGS300

I went outside into the bright sun. I disconnected the battery and waited 10 minutes. I then reconnected it and started the car. The check engine light comes on and code 21 is still thrown. All I did was change the alternator. The EFI fuse is fine. What should I check?

justfixit

Engine code 21 is for heater circuit of bank #1 oxygen sensor
Air Conditioner code 21 is for solar sensor which should be checked outdoors or with a flash light on sensor.
Your condition should have nothing to do with alternator replacement. You should check resistance of sensor as shown below. Bank #1 is the sensor closest to radiator and before the catalytic converter.