P0598: Thermostat heater ground-short status is distinct from an open circuit

The P0598 check in GM's GMDAT table addresses the low, ground-short direction of thermostat-heater control monitoring. It evaluates the electrical output stage rather than directly measuring thermostat movement or coolant flow.

Driver state explains the low reading

The source describes low voltage during the driver's open state. That observation alone can accompany either a ground short or an open circuit; the output driver's status byte is used to classify the ground-short fault. The separate P0597 code handles the open-circuit classification.

This distinction matters when a scan tool shows only the generic low label. A single voltage value cannot replace the controller's command-state and diagnostic-status interpretation. The exact service circuit test is needed to separate the possible electrical causes.

Qualification and verification

The test requires a valid run/crank state, an engine that is not cranking and suitable ignition conditions. A relevant previous circuit-test result must also be determinate. The documented counter is 15 failing samples out of 30, at one-second intervals.

Keep those qualifications in mind if the code does not return immediately after clearing. The monitor may not yet have made a valid decision. Verify restored circuit behavior and a completed test after repair. If coolant temperature still behaves incorrectly, investigate that separately; an electrical heater code does not establish whether the thermostat is mechanically regulating temperature correctly.

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.

GM diagnostic parameters, PDF page 40.