Fix P0599: Mechanic’s solutions that WORKED

The P0599 code indicates a malfunction in the engine coolant thermostat heater control circuit, specifically signaling a “high” input problem. This code is important because it relates to the vehicle’s ability to regulate engine temperature through the thermostat’s electric heating element. Proper functioning of this component is crucial for optimal engine performance and to prevent overheating.

P0599 quick overview

Meaning	P0599: Engine Coolant Thermostat Heater Control Circuit High
Is it serious?	Yes, it can lead to engine overheating and significantly affect vehicle performance.
Possible causes	– Faulty thermostat – Wiring or connector issues in the thermostat circuit – Faulty engine control module (ECM)
How to diagnose?	Inspect the thermostat and its connectors for damage or corrosion Test the thermostat’s heater resistance to ensure it’s within specifications Check the electrical wiring for shorts, opens, or poor connections Verify ECM operation, ensuring it properly controls the thermostat heater circuit

How to fix P0599 – learn from mechanics

I looked into P0599 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: BMW

Vehicle Information 2004 BMW X3 3.0i 3.0L

Issue + Repair History

• MIL illuminated.
• Code P0599 for heater circuit in the electric heating element.

Other OBD2 Codes + Test Results

• P0599 –

Mechanic Suggestions

• Replace the thermostat for this code; it is one of those electric thermostats.
• Be careful when removing attaching hoses; if the connector breaks, only repair is a new hose at a higher cost.

Final Fix

• Ordered a new unit from BMW for the electric heating element.
• Cost approximately $120.00 with shipping.
• Customer reported no MIL lights returning after four days.

Case #2: Volkswagen

Vehicle Information 2003 Volkswagen Passat W8 4.0L

Issue + Repair History

• Check engine light on and fan staying on for too long (about 10 minutes) after engine is turned off.
• Code P0599 stored in system for the electric thermostat circuit too high.
• Checked power to thermostat with key on engine off and found 12 volts on one wire.

Other OBD2 Codes + Test Results

• P0599 – Thermostat control circuit high

Mechanic Suggestions

• Replace the thermostat assembly to fix the issue; fixing/clearing the fault should prevent the fan from coming on during engine off.
• Suspect the thermostat unit is faulty; reading is outside the resistance spec the ECM is looking for.
• Consider checking for continuity between signal return and the PCM; there might be an open in the signal return or PCM does not recognize the signal.

Final Fix

• Replaced the electric thermostat, which resolved the issue.
• Cost approximately $450 with gasket.

Case #3: Chevrolet

Vehicle Information 2013 Chevrolet Sonic LT 1.8L

Issue + Repair History

• Check engine light on.
• Fan staying on for too long (about 10 minutes) after engine is turned off.
• Code P0599 stored in system for the electric thermostat circuit high voltage.
• Computer constantly outputting 100% to the thermostat heater regardless of engine temperature.
• Tested thermostat heater resistance: 3 ohms at cold engine, 5 ohms at hot engine.

Other OBD2 Codes + Test Results

• P0599 – Engine coolant thermostat HEATER control circuit high voltage
• Electric thermostat resistance check: @200 degrees F – 6 ohms, at room temp – 14 ohms

Mechanic Suggestions

• Replace the thermostat assembly; the heater circuit should have constant power on one wire, and the control wire is pulse-width modulated by the computer.
• Thermostat resistance might be too low; compared to a new one which was 15 ohms.
• Verify power feed and verify pulsed ground control (using a scope). If there’s voltage and ECM control, replace the thermostat. If not, verify wiring.
• Low coolant can cause this DTC; chances are low coolant damaged the thermostat heater circuitry.
• Check for a bad connection electronically, not just visually.
• Use a test light across the ignition and ground circuits and command the thermostat heater to 100% and then 0% with the scan tool. The test light should be on at 100% and off at 0%.

Final Fix

• Replaced the thermostat, which resolved the issue.

Diagnostic Step	Brief Explanation
Replace the thermostat assembly	Directly addresses the fault in the thermostat heating element
Test thermostat heater resistance	Ensures the heating element operates within the specified range
Inspect electrical connections and wiring	Identifies potential electrical issues affecting thermostat control
Verify ECM control of the thermostat heater circuit	Confirms the ECM is properly managing the thermostat’s operation
Check coolant levels	Low coolant can indirectly cause or exacerbate this code
Use a scope to verify pulsed ground control	Ensures the ECM is correctly modulating control to the thermostat

In each service case, replacing the thermostat assembly was the key to resolving the P0599 issue. This underscores the importance of diagnosing and addressing any malfunction within the thermostat’s control circuit promptly to prevent engine overheating and ensure the vehicle operates safely and efficiently.