Diagnostic article Generic OBD-II
P066B: cylinder 1 glow output short toward power
P066B monitors cylinder 1's glow-plug circuit for a short to high voltage.
Read before applying this article
Documented application
2025 GM 25OBDG04B diagnostic parameter tables; the cited monitor only. Vehicle, engine and calibration applicability must be confirmed separately.
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.
P066B monitors cylinder 1's glow-plug circuit for a short to high voltage. The cited hardware strategy changes its detection impedance according to the attached load resistance.
The load band changes the criterion
When load resistance is above 0.65 ohm, the source uses a power-short path below 0.5 ohm. For a load between 0.2 and 0.65 ohm, it uses a path below 0.14 ohm. Neither number is a universal healthy-glow-plug resistance specification; each belongs to a particular short-detection case.
This dependence means that a resistance observation without identifying the load condition cannot fully reproduce the monitor's reasoning. The circuit-high label also does not independently prove that the glow element itself overheated.
Conditions and confirmation
The test runs with the engine operating outside cranking, battery voltage in range, no glow-system supply fault and diagnostics enabled. It filters ten failures among twenty samples on a 100 ms task and lists Type B reporting plus glow-lamp indication.
Verification should establish normal cylinder-1 circuit behavior under the relevant enabled conditions and use the service procedure to separate the power path from the load. The source does not identify a pin location or select a replacement component. Do not treat its impedance criteria as permission to bridge supplies, apply external battery voltage or test a live circuit with an ohmmeter.
Source and applicability
2025 GM 25OBDG04B diagnostic parameter tables; the cited monitor only. Vehicle, engine and calibration applicability must be confirmed separately. These tables describe detection logic, not a complete repair procedure or a component replacement verdict. gm-2025-04b, PDF page 1233.
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 Control Circuit High
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