Diagnostic article Generic OBD-II
P06C6: why cylinder 2 can fail the glow-plug rationality test
P06C6 is a cylinder 2 glow-plug operating-resistance plausibility fault in the documented GM monitor.
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Documented application
Documented 2025 GM diagnostic parameter monitor in 25OBDG04B.pdf; model, engine and calibration applicability require vehicle-specific confirmation.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P06C6 does not, by itself, prove that the wrong cylinder 2 glow plug has been fitted. GM's reviewed implementation describes a resistance rationality monitor: measured voltage and current are used to calculate whether the operating plug remains within a calibrated range.
What the controller must see
The table displays a resistance-parameter window from 0.23 to 2.00 and detects values outside it. Because the unit is not explicitly attached to that expression on the source page, it should not be republished as an unconditional ohmmeter specification. The calculation also depends on voltage and current measured during operation.
Before the test is allowed, cylinder 2 must be commanded on for the required calibrated time. The individual glow-plug electrical check and the voltage-feedback circuitry must be free of relevant faults. A voltage-feedback problem can therefore prevent a useful rationality result, even when a disconnected plug measurement appears normal.
Confirmation requires an enabled run
Other conditions include suitable battery voltage, an enabled diagnostic system and the engine running outside cranking. Failure accumulation is 15 of 25 samples on a 100-millisecond task, with Type B, two-trip reporting.
Review powered operating data and monitor completion rather than interpreting one stored code as a component verdict. A brief crank or an on-command that ends too soon does not necessarily repeat this test. This discussion is limited to the 2025 GM parameter file; exact engine coverage, correct plug selection and the isolation procedure must come from the vehicle's service information.
Source: GM monitor, PDF page 1251.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Cylinder 2 Glow Plug Incorrect
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT