I recenty bought a new (for me) 93 4x4. 209k but the previous owner rebuilt the motor <5000k ago. the check engine lamp was burned out when I got it, I put a new one in and found a code 21 and 25. I checked underneath, found the wiring for the o2 sensor a little funky, so I spliced in a new section from a wrecker. also replaced the sensor with a new bosch unit. it's the 4-wire warmed version. I'm positive I wired everything correctly. now the code 25 is gone but the 21 is persistent. I measured the voltage from E1 to Vf1 and didn't get anything, no matter what I did. the FSM troubleshooting flowchart says to replace the ECM under that condition. I've never encountered a bad ECM before, so I'm wary of that. anyone have any ideas where to look next? is there a way to test the ECM, other than trying a different one? thanks...

 

What is 21? Whats the description?

 

code 21 is : open or short in : main heater circuit of oxygen sensor or main circuit of oxygen sensor, or failure of heater circuit or oxygen sensor. or ECM.

 

so check for voltage on the O2 heater wires. If you've got voltage there, and a new O2 sensor, then maybe it is the ECU.