Thermostat Heater Control Circuit Low
The P0598 code indicates that the powertrain control module (PCM) has detected a low voltage condition in the thermostat heater control circuit. This circuit is responsible for heating the engine thermostat to regulate coolant temperature more precisely, improving fuel economy and emissions. A low voltage reading suggests a short to ground, open circuit, or faulty component in the heater circuit.
1. Most Common Causes
- Faulty electronic thermostat (integrated heater) with low internal resistance
- Heater circuit wiring shorted to ground
- Corroded, oxidized, or damaged pins in the thermostat connector
- Excessive resistance in the wiring due to splices or deteriorated cables
- Internal ECM/PCM failure (less common but possible)
- Blown heater circuit fuse or incorrect low-amperage fuse
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Engine may run cooler than normal or take longer to reach operating temperature
- Reduced fuel economy
- Possible poor heater performance
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm P0598 and check for related codes.
Step 2: Visually inspect the thermostat heater connector and wiring for damage, corrosion, or loose connections.
Step 3: Measure resistance of the thermostat heater element; compare to manufacturer specs (typically 5-20 ohms).
Step 4: Check for voltage at the heater connector with the ignition on; should be battery voltage. If low, trace circuit for shorts or opens.
Step 5: If all checks pass, suspect a faulty PCM; perform a PCM test or reprogram.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, but it may cause reduced fuel economy and longer engine warm-up. It's best to repair promptly to avoid potential overheating or emissions issues.
How much does it cost to fix?▼
Typically $50-$150 for parts and labor, depending on whether the thermostat assembly or just the heater element needs replacement.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.