What Is Code P0599?
Code P0599 is a diagnostic trouble code (DTC) that signals a problem with your vehicle’s thermostat heater control circuit. Specifically, it means the engine control module (ECM) has detected a voltage or current level that is higher than expected in the circuit that controls the electric heater element within the thermostat housing. This code is most commonly found in European vehicles and indicates a cooling system malfunction that should be diagnosed and repaired promptly.
What Does P0599 Mean?
The thermostat heater control circuit is responsible for managing an electric heating element that helps regulate engine coolant temperature more precisely, especially during cold starts and low-load conditions. When the ECM detects excessive voltage or current in this circuit—typically above the normal operating range—it sets code P0599.
The “high” designation means the circuit is receiving more electrical power than the ECM expects. This could indicate:
- A short circuit in the heater element itself
- Damaged wiring or connectors causing electrical resistance changes
- A faulty thermostat heater assembly
- An issue with the ECM’s control signal or driver circuit
This code is primarily associated with European manufacturers like BMW, Mercedes-Benz, Audi, Volkswagen, and Porsche that use electronically controlled thermostats with integrated heating elements.
Common Symptoms
Drivers may experience the following symptoms when code P0599 is present:
- Check Engine Light: The primary indicator that code P0599 has been set
- Engine Temperature Fluctuations: Coolant temperature may fluctuate more than normal or not reach optimal operating temperature
- Poor Fuel Economy: The engine may run less efficiently if it cannot maintain proper operating temperature
- Sluggish Cold Starts: The vehicle may be slow to warm up or hesitate during initial acceleration when cold
- Reduced Heater Performance: The cabin heater may not produce warm air as quickly as expected
- No Drivability Issues: In many cases, the vehicle drives normally despite the fault code being set
Possible Causes (Ranked by Frequency)
1. Faulty Thermostat Heater Element
The most common cause of P0599 is a defective electric heater element within the thermostat assembly. Over time, the heating coil can develop internal shorts or resistance changes that cause excessive current draw, triggering the high-voltage fault code.
2. Damaged Wiring or Connectors
Corroded, pinched, or damaged wiring in the thermostat heater circuit can create resistance anomalies or short circuits. Similarly, loose or corroded connectors can cause intermittent electrical faults that the ECM interprets as a high-voltage condition.
3. Water Intrusion or Corrosion
Moisture entering the thermostat housing or electrical connectors can cause corrosion and electrical leakage, resulting in abnormal voltage readings. This is particularly common in vehicles exposed to salt, humidity, or water crossings.
4. ECM or Control Module Malfunction
Less commonly, the ECM itself or its thermostat heater control driver circuit may malfunction, causing it to misinterpret normal circuit conditions as excessive voltage. This would require module reprogramming or replacement.
5. Coolant Temperature Sensor Fault
A faulty coolant temperature sensor (CTS) can send incorrect readings to the ECM, causing it to command excessive heater current or misinterpret the circuit voltage, indirectly triggering code P0599.
6. Blown or Weak Battery
A weak or failing battery can cause voltage fluctuations that the ECM detects as abnormal in the thermostat heater circuit, though this is a less common cause.
Diagnostic Steps
Step 1: Retrieve and Document the Code
Use an OBD-II scanner to read the code and note any additional freeze frame data, such as engine load, temperature, and RPM when the fault occurred. This information helps pinpoint the exact conditions triggering the code.
Step 2: Perform a Visual Inspection
Inspect the thermostat housing, heater connectors, and related wiring for:
- Visible corrosion or oxidation on connectors
- Damaged, pinched, or melted wiring
- Loose connections
- Signs of coolant leaks around the thermostat
Step 3: Check Connector Condition
Disconnect the thermostat heater connector and inspect the pins for corrosion. Clean with electrical contact cleaner if needed. Reconnect firmly and clear the code to see if it returns.
Step 4: Test Circuit Voltage and Resistance
Using a multimeter, measure:
- Voltage: Check the voltage at the thermostat heater connector with the engine running. Compare to manufacturer specifications (typically 12-14V for normal operation).
- Resistance: Measure the resistance of the heater element itself. A short circuit will show very low or zero resistance; an open circuit will show infinite resistance. Normal values vary by manufacturer but are typically 5-50 ohms.
Step 5: Inspect Wiring Harness
Trace the entire thermostat heater circuit wiring from the ECM to the thermostat. Look for:
- Abrasions or cuts in the insulation
- Pinch points where wires may be compressed
- Water or coolant contamination
Step 6: Check Coolant Temperature Sensor
Test the coolant temperature sensor resistance at various temperatures. Faulty sensors can cause the ECM to misinterpret circuit conditions. Compare readings to manufacturer specifications.
Step 7: Clear Code and Monitor
After performing repairs or cleaning connections, clear the code and test drive the vehicle under various conditions (cold start, highway, idle) to confirm the fault does not return.
Repair Cost Estimates
DIY Repairs
- Connector Cleaning: $0-20 (just cleaner and contact spray)
- Wiring Repair: $20-100 (wire, connectors, solder, heat shrink)
Professional Repairs
- Thermostat Heater Element Replacement: $300-800 (parts + labor)
- Complete Thermostat Assembly Replacement: $400-1,200 (parts + labor)
- Wiring Harness Repair/Replacement: $200-600 (parts + labor)
- Coolant Temperature Sensor Replacement: $150-400 (parts + labor)
- ECM Reprogramming or Replacement: $500-1,500+ (rare, only if module is faulty)
Note: European vehicles (BMW, Mercedes, Audi, VW) typically have higher labor rates and parts costs than domestic vehicles. Dealership repairs will be more expensive than independent shops.
Can I Still Drive?
Severity: Low
Code P0599 is generally considered a low-severity fault. In most cases, your vehicle will continue to operate normally with no immediate safety risk. However, you should address it within a reasonable timeframe (within a few weeks).
Driving Safely with P0599
- Safe to Drive: Yes, the vehicle is generally safe to drive to a repair shop or mechanic.
- Monitor Temperature: Keep an eye on the engine temperature gauge to ensure the engine doesn’t overheat.
- Avoid Extreme Conditions: Don’t tow heavy loads or drive in extreme heat until the code is resolved.
- Check Coolant Level: Regularly check coolant levels, as the fault could indicate a leak.
- Don’t Ignore: While not immediately dangerous, prolonged operation with this code could lead to inefficient cooling and potential engine damage over time.
Frequently Asked Questions
Q: What is the difference between P0599 and other thermostat codes?
A: P0599 specifically indicates a “high” voltage or current condition in the thermostat heater circuit. Other related codes include P0597 (thermostat heater control circuit low), P0598 (thermostat heater control circuit open), and P0600 (thermostat heater control circuit malfunction). Each code points to different electrical faults within the same system.
Q: Can I drive with the check engine light on?
A: Yes, you can drive with P0599 active. The code itself is not critical and won’t cause immediate engine damage. However, you should have it diagnosed and repaired within a few weeks to prevent potential cooling system issues and maintain fuel efficiency.
Q: Is the thermostat heater the same as the engine thermostat?
A: No. The thermostat heater is an electric heating element integrated into or near the thermostat housing, used to improve temperature control and warm-up performance. It’s separate from the mechanical thermostat valve that regulates coolant flow. European vehicles commonly use this technology for emissions and efficiency compliance.
Q: Will replacing the thermostat fix P0599?
A: Possibly. If the fault is caused by a defective heater element within the thermostat assembly, replacing the entire thermostat (which includes the heater) will resolve the code. However, if the problem is wiring, connectors, or the ECM, replacing just the thermostat won’t help. Proper diagnosis is essential before ordering parts.
Q: How much does a thermostat replacement cost?
A: For European vehicles, expect $400-1,200 in total cost (parts and labor) for a complete thermostat assembly replacement. Independent shops may charge $300-800, while dealerships typically charge $800-1,500 or more. Labor time is usually 1-3 hours depending on vehicle model and engine bay layout.