Diagnostic article Generic OBD-II
P0599: Thermostat heater circuit remains high in the commanded state
P0599 identifies a high thermostat-heater control-circuit condition using output-driver feedback.
Read before applying this article
Documented application
2009 General Motors diagnostic parameter file 09OBDG01 GMDAT Engine Diagnostics.pdf. Section-specific modules and calibrations; no per-code model or engine applicability established.
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.
P0599 is the high-circuit member of the thermostat-heater diagnostics in GM's GMDAT summary. The source describes voltage remaining high during the driver's closed state and classifies the result through the output driver's power-short status.
The command makes the voltage meaningful
A high voltage is not inherently a fault in every operating state. Here, the diagnostic significance comes from seeing it when the driver state calls for a different response. That is why recording the heater command and driver feedback together is more useful than reporting a voltage without context.
The neighboring P0597 and P0598 entries classify open and ground-short conditions. They are separate electrical decisions, not three interchangeable descriptions of poor coolant warm-up.
A valid circuit check has prerequisites
The controller requires the engine not to be cranking, an active run/crank state and ignition in range. A related previous circuit-test result must not be indeterminate. The published evaluation uses 15 failures in 30 one-second samples, so an immediate restart may not complete verification.
Use the applicable wiring and driver test procedure to identify the source of the high feedback. The table does not prove that the thermostat assembly itself is the failed part. Following correction, verify completed electrical monitoring and then assess any remaining temperature-control symptoms on their own evidence. Clearing the code does not demonstrate restored heater control.
Source and applicability
2009 General Motors diagnostic parameter file 09OBDG01 GMDAT Engine Diagnostics.pdf. Section-specific modules and calibrations; no per-code model or engine applicability established. This explanation describes the documented monitor, not a universal repair procedure.
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
Thermostat Heater 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