P06C9: cylinder 5 glow-plug rationality with a source-label caveat

P06C9 is normally described as “Cylinder 5 Glow Plug Incorrect.” The reviewed GM row also identifies its system as cylinder 5 electrical-resistance rationality. However, the malfunction-description cell refers to glow plug 4. That internal inconsistency prevents treating every word of the row as confirmed cylinder-specific guidance.

What remains supported

The monitor strategy calculates glow-plug resistance from measured voltage divided by measured current and compares it with a calibrated operating range. It displays a parameter window of 0.23 to 2.00 without an explicit unit beside the expression. These are monitor facts, not sufficient instructions for testing a removed component.

The test requires an enabled glow system, a commanded-on interval, no individual electrical fault and no voltage-feedback fault. The engine must be running outside cranking, battery voltage must be in range, and diagnostics must remain enabled. Failure accumulation uses 15 out of 25 samples at a 100-millisecond task rate; reporting is Type B over two trips.

How the inconsistency affects diagnosis

Do not use the isolated “plug 4” sentence to redirect work from cylinder 5 to cylinder 4. Confirm the cylinder assignment in the applicable service diagnostic before any component-specific intervention. Separately, verify that the commanded-on and feedback prerequisites were satisfied; a disabled rationality check cannot confirm a repair.

The supported conclusion is an operating-resistance plausibility failure, not proof of an incorrect replacement part. Model and engine applicability remain unverified beyond this 2025 GM file, and the cylinder wording requires additional source review.

Source: GM monitor, PDF page 1254.