GM Service Manual:
DTC P1558 set indicates that an improper voltage level has been detected on the signal circuit which controls the cruise control inhibit circuit.
• The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
Step 1:
Turn OFF the ignition.
Disconnect the PCM harness connector.
Turn ON the ignition, with the engine OFF.
Using a DMM connected to a good ground, measure the voltage of the cruise control inhibit signal circuit. (B+... = "90V DC"?)



  1. If 90VDC, go to Step 2. If not, go to Step 3.

    Step 2:
    Turn OFF the ignition.
    Disconnect the cruise control module.
    Turn ON the ignition, with the engine OFF.
    Probe the cruise control inhibit signal circuit with a test lamp that is connected to a good ground.

    If it doesn't light up, inspect for poor connections at the harness connector of the PCM. If it lights up, inspect for poor connections at the harness connector of the cruise control module.

    Step 3.
    Test the cruise control inhibit signal circuit for a open, high resistance or short to ground.
    If the condition is there, inspect for poor connections at the harness connector of the cruise control module. Else, clear the DTC.