Have a 93 JXL Sidekick that is giving me a 21 error code. Replaced the throttle positon sensor, adjusted it. After open loop warm up, it goes to closed loop error again. Checked all emissions sensors and they are all within specs. Attempted to check harness but wiring too tight under dash. Car runs great before warm up, but after warmup, it goes into default mode, with no spark advance.
Wondering if there is any other way of checking before I have to buy a new computer? Or is there a way of forcing it to stay open loop? Any help would be appreciated.

 

What about the O2 sensor? You said it runs great till its warmed up*shrug*