Diagnostic article Generic OBD-II
P06C5: cylinder 1 glow-plug resistance rationality
P06C5 means the monitored cylinder 1 glow-plug resistance falls outside the documented operating range.
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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.
The common P06C5 label says “Cylinder 1 Glow Plug Incorrect.” In the reviewed GM table, the actual test is an electrical-resistance rationality check. It asks whether cylinder 1's glow plug continues to operate within the specified range, rather than directly identifying an incorrect part number.
An operating calculation
The controller calculates resistance from measured voltage divided by measured current. The displayed calibration window is 0.23 to 2.00 for the glow-plug resistance parameter. The source does not explicitly print a unit alongside that expression, so these values should not become an assumed workshop specification.
This is a loaded operating calculation. A cold resistance reading taken with the component disconnected does not necessarily reproduce the conditions under which P06C5 was set. Likewise, “incorrect” alone does not prove that someone installed the wrong plug.
The monitor first needs trustworthy inputs
The glow plug must be commanded on for a calibrated interval, and the system must have no individual electrical fault or glow-plug voltage-feedback fault. Battery voltage must be in range. The engine-running requirement excludes cranking, and diagnostics must be enabled.
GM specifies 15 failing samples within 25 samples at a 100-millisecond task rate, with Type B reporting over two trips. For verification, establish that the powered resistance check completed and that prerequisite circuit or feedback faults were absent. Repeating a start without meeting the commanded-on requirement may not exercise it. Vehicle-specific service information is still needed to identify the correct component and applicability; this monitor page does not supply those details.
Source: GM monitor, PDF page 1250.
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 1 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