P06C8: cylinder 4 glow-plug incorrect versus an electrical short

P06C8 is the cylinder 4 glow-plug resistance rationality code in the GM source reviewed here. Unlike a circuit high or low diagnosis, it evaluates whether the powered plug's calculated resistance remains within the manufacturer's calibrated operating range.

What “incorrect” establishes

The dictionary's “glow plug incorrect” wording should not be read as a confirmed parts-identification result. The documented evidence is a ratio of measured voltage to measured current outside the accepted resistance window. GM displays that window as 0.23 to 2.00; an explicit unit is absent beside the expression, so the page does not justify a universal bench-test limit.

The distinction from an electrical short is also visible in the enable rules. The individual glow-plug electrical fault must be absent, the glow system enabled and the plug commanded on for a calibrated time. Voltage-feedback circuitry must also be fault-free. Those are prerequisites, not additional conditions that themselves set P06C8.

Interpreting a retest

The engine must run outside cranking with battery voltage in range and diagnostics active. The table calls for 15 failing samples in a 25-sample window at 100-millisecond updates and lists Type B, two-trip reporting.

Confirmation therefore requires more than reconnecting the plug and observing no immediate code. Establish that the powered rationality check became eligible and completed. The source does not establish a faulty part, a repair method, or universal cylinder-location information. Exact vehicle and engine applicability require service information beyond this 2025 monitor summary.

Source: GM monitor, PDF page 1253.