P067D: understanding the cylinder 5 glow-plug high fault

P067D concerns the cylinder 5 glow-plug output, which the reviewed GM diagnostic table checks for electrical integrity during operation. The high-circuit description refers to a power-short signature at the controller connection. It is not a direct measurement of the plug's heating performance.

Reading the thresholds correctly

The power-short threshold depends on the load. Above 0.65 ohm of load resistance, the controller looks for less than 0.5 ohm of impedance to a qualifying voltage source. For a load between 0.2 and 0.65 ohm, the corresponding limit is less than 0.14 ohm. The qualifying source can be within the normal or high operating-voltage range.

Thus “high” does not require the whole vehicle electrical system to be overvoltage. A connection to an otherwise normal supply can satisfy the documented mechanism. The two impedance values also describe different load cases, so they are not interchangeable pass/fail specifications.

Establishing a meaningful result

The table excludes cranking and requires the engine to be running with suitable battery voltage. A glow-system supply fault or disabled diagnostic prevents this check from being evaluated as described. Ten failures within 20 samples, updated every 100 milliseconds, support Type B reporting over two trips and a glow-lamp request.

Before treating a post-repair absence of P067D as confirmation, establish that the cylinder 5 monitor actually ran under those conditions. This article describes the 2025 GM file's monitor; exact engine applicability, connector identification and component-level testing remain vehicle-specific.

Source: GM monitor, PDF page 1247.