91 GMT400 kicking my rear!

[Spenj0rk]

I have this 91 c1500 that is giving me a fit.
Problem:
Dim fast flashing CEL and really high idle. I have replaced distributor, set timing, replaced bad vacuum hoses, installed new IAC, CTS, TBS, cleaned all ground cables, repaired bad wires, pulled ecm, cleaned terminals and reset PROM. as soon as you start it, it idles a little high like it should. The engine light faintly flashing very fast then the idle shoots up to around 2k rpms. Then, one day it will run perfect with no CEL light on. Then the next day I have the same problem with fast idle and flashing CEL. When jumping A&B I get code 12x3 then code 15 but I have replaced the CTS. Any help would be great.

 

[Drunkcanuk]

Well that's a heck of a parts cannon you fired at it.
Did you do any ACTUAL diagnostics other then flashing the CEL?
Did you test the system that the code was indicating?
Just because it has a problem with the O2 sensor, doesn't mean the O2 sensor is the problem.... hopefully that makes sense.
The sensor could be fowled because it's running rich, but WHY is it running rich and destroying the sensor.

Do you have access to a OBD1 scanner that can look at data real time? I'm sure you don't but have to ask anyway.

Go to the engine and performance section, go down to the stickys, there is a section with the Factory Service Manuals to download. Find your year and download it.
There is a ton of information in there, including how to PROPERLY diagnose the codes you are getting from the CEL. That way you aren't just tossing cash/parts and hoping it helps.

 

[95 Tahoe]

You can download tunerpro and use an adapter cable for the OBD1. That's what I do. You can look at the sensor output for the ECT. Usually they will run with a bad ECT but poorly.