P0599

Code

Generic P — Powertrain

Thermostat Heater Control Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 22 EN: 83 RU: 38

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Page language: EN

Causes

Short to battery (B+) on thermostat heater control wiring
Corroded, pushed-back, or damaged connector pins at thermostat or ECM
Failed thermostat with internal heater shorted
Blown or incorrect fuse or stuck relay supplying constant battery voltage
ECM/PCM driver output failed (stuck high)
Aftermarket wiring or recent repairs that disturbed the harness

Symptoms

Check Engine Light (MIL) illuminated
Long or abnormal engine warm-up (engine takes longer to reach operating temperature)
Reduced fuel economy or poor idle until engine warms
Cooling system not regulating properly (thermostat may fail-open or fail-closed depending on fault)
Often no overt driveability symptoms other than MIL

What to check

Read freeze-frame/fault data and confirm P0599 is current or stored; check related codes
Visually inspect wiring and connectors for chafing, heat damage, corrosion, or melted insulation between thermostat and ECM
Check fuse(s) and related relays for the thermostat heater circuit
Measure battery voltage at battery and at thermostat connector with key ON/engine OFF
Unplug thermostat connector; see if code changes (use caution and follow manufacturer procedures)
Measure resistance of thermostat heater element (manufacturer spec) and check for short to B+ or ground

Signal parameters

Expected: control output normally driven low/ground or pulsed (PWM) when active; voltage near 0 V when commanded off or pulsed as spec
High condition: circuit voltage near battery voltage (~12 V) or equal to B+ when it should be low
Resistance of thermostat heater element: typically low ohms (check vehicle-specific spec); infinite or very high = open, near 0 ohms with short to B+ = problem
With connector unplugged, ECM control output may show switching (0–~12 V or ground switching) depending on design; consult service manual

Diagnostic algorithm

Retrieve freeze-frame and all related codes; record operating conditions when fault set.
Visually inspect the thermostat heater, harness, and connectors for damage, corrosion, or signs of arcing.
Check fuses/relays that supply the thermostat heater for correct rating and operation.
With key ON (engine OFF), back-probe thermostat heater control pin and measure voltage. Compare to expected (should not be constant B+ if ECM is controlling it).
Disconnect the thermostat connector and re-check for voltage on the harness side. If voltage disappears, the fault may be in the thermostat; if voltage remains, suspect a short to B+ in the harness or ignition-switched supply.
Measure resistance of the thermostat heater element to ground and to the control pin; compare to spec. Check for short to battery or ground.
If wiring appears intact but control signal is present and high, test/verify ECM output using a scope or substitute known-good ECM (if available) per manufacturer procedures.
Repair wiring or connector faults (repair/replace harness, apply proper crimps/terminals, replace grommets), replace thermostat assembly if internally shorted, replace fuse/relay if defective.
After repair, clear codes, perform a cold-start and warm-up verification, and road test to confirm the code does not return.

Likely causes

Short to B+ on the heater control conductor between thermostat and ECM
Thermostat heater assembly internally shorted to battery
Loose or corroded connector at thermostat allowing intermittent high readings
Faulty ECM output transistor or control circuitry

Fault status

⚠️ Status

Thermostat Heater Control Circuit High — control circuit voltage higher than specified (possible short to battery, connector/harness fault, or failed thermostat/ECM). MIL set.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.5 hours