Probably a short or loose connection.
Per the 92/93 FSM, the fan relays control signals are switched on off by switching the ground leg inside the pcm based on input from the temp sensor.
If the fans are on, then the pcm ground is in good working order.
There are a pair of wires linking the bcm and pcm. Check the wire harness at the pcm to make sure they are tight and dry.
You can further diagnose which wire and possible cause via the 6th post
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Once again, this is all based on 92. The 91 should be the same though.
Per the 92/93 FSM, the fan relays control signals are switched on off by switching the ground leg inside the pcm based on input from the temp sensor.
If the fans are on, then the pcm ground is in good working order.
There are a pair of wires linking the bcm and pcm. Check the wire harness at the pcm to make sure they are tight and dry.
You can further diagnose which wire and possible cause via the 6th post

1992 DeVille Intermittent Service Engine/Service Vehicle...
I have an intermittent 'Service Engine/Service Vehicle' light problem with my 1992 DeVille 4.9. About every 100 miles, the 'Service Engine Soon' light comes on, and then the 'Service Vehicle Soon' light also comes on. These lights will stay on for a few moments, then one or the other will go...
Once again, this is all based on 92. The 91 should be the same though.