Ok, a word of warning:

Most people that own FSM's (factory service manuals) can't do this the first one or three times they try. Most won't attempt it without the manual in front of them. What I'm going to try to do here is translate the procedure into terms that you can print or write down and take out to the car, and actually make it work. If it doesn't work for you, it's simply one of those things thats difficult to begin with using the manual, and probably worse with my translation. This will ONLY work on 92-93 Bonnevilles with the electronic climate control (digital).


To enter Diagnostic mode, turn the ignition on and press OFF and WARM buttons simultaneously. This is where the FSM tries to screw you first. There is no WARM button. It's the temperature INCREASE button (up). You will now enter diagnostic mode with a segment test (display test).

Now that you're in diagnostic mode, OFF backs up one level, Fan DOWN scrolls through options/levels. AUTO terminates diagnostics at any time.

If you have no faults, 00 will be displayed. Ignore 01, 02, and 03 if you hit fan down, it takes you places in diagnostics that I'm not covering here. From there, you can do MANY things including testing fan PWM control settings, coolant temperature, in vehicle temp, outside ambient temp, solar sensor level, ignition voltage, airmix valve position commanded and actual, and other crap. Not important though. What you need is diagnostics, and you've probably got a code or two if you're reading this article. The codes should scroll through automatically (I'm assuming, as I've only ever had one at a time, and you probably only have one). If codes are 3 digits starting with a 1, they're a history code that has tripped in the last 50 ignition cycles but is not current (fixed itself). This probably isn't the case with your display symptoms. If codes are 2 digits, they are current. Codes are here remember the history code thing, code 10 would be 110 if it was an old code):

00: no codes present
10: ambient temp sensor circuit open or shorted. This is the sensor out in front of the radiator. Sensor, connections, or wiring.
13: in-vehicle temp sensor circuit open or shorted. This is behind the slits in the cluster trim near your cig lighter. Sensor, connections, or wiring.
15: LH solar sensor circuit open or shorted. This is the little dome on the dash. The little round flat one is for your headlights. Solar load sensing adjusts your AC for you in the summer.
35: E&C data line failure with heater and AC control assembly. Bad connection or wiring between HVAC programmer (above passenger footwell) and ECC head unit.
38: Serial data line failure with PCM. This would be connections or wiring between HVAC programmer and PCM.
40: Driver air mix motor circuit open or shorted, or not calibrated.
48: Long term freon loss. This gets me once a year, and is typically bogus.
52: Keep alive memory lost, sets with battery disconnected
66: Low freon




There's a TON more to this diagnostic procedure. I've covered maybe 10% of it here. I'll update this article soon with the procedure to clear codes.

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