P0599
UniversalThermostat Heater Control Circuit High
Medium SeverityThis code indicates that the Powertrain Control Module (PCM) has detected a higher-than-normal voltage in the control circuit for the thermostat heater. This electrical issue can prevent the engine from reaching its optimal operating temperature, potentially leading to reduced fuel efficiency and increased emissions.
⚠️ Before You Begin (Safety First)
Always ensure the engine is cool before working on cooling system components. Disconnect the negative battery terminal before performing any electrical work to prevent accidental shocks or damage to the vehicle's electrical system. Wear appropriate personal protective equipment, including safety glasses and gloves.
Common Symptoms
- Check Engine Light illuminated
- Engine running cooler than normal
- Delayed engine warm-up
- Reduced fuel economy
- Heater may not work efficiently
Common Causes
- Short to voltage in the thermostat heater control circuit
- Faulty electronic thermostat heater element
- Damaged or corroded wiring or connectors
- Faulty Powertrain Control Module (PCM) (rare)
Frequently reported on: Chevrolet Cruze 2011-2016, Ford Focus 2012-2018, Hyundai Elantra 2011-2016, Volkswagen Jetta 2005-2014, General/Universal
Diagnostic Steps
- Inspect the wiring and connectors leading to the electronic thermostat for any signs of damage, corrosion, or shorting to voltage.
- Using a multimeter, test the thermostat heater control circuit for proper voltage and continuity according to your vehicle's service manual.
- If wiring appears good, test the resistance of the thermostat heater element itself. Compare readings to manufacturer specifications.
- If the thermostat heater element is faulty, replace the electronic thermostat assembly.
- Clear the P0599 code and perform a test drive to confirm the repair.
👨🔧 When to Call a Professional
If you have replaced the thermostat and checked the wiring, and the P0599 code persists, it's time to consult a professional technician for advanced electrical diagnosis. A persistent P0599 might indicate a more complex wiring harness issue or a faulty Powertrain Control Module (PCM).
Commonly Asked Questions
What does P0599 mean?
P0599 means the engine's computer (PCM) detected a higher-than-expected voltage in the circuit that controls the electric heater on your engine's thermostat.
Can I drive with a P0599 code?
While you can often drive with this code, it's not recommended long-term. Your engine may run too cool, leading to poor fuel economy, increased emissions, and potentially reduced engine lifespan over time.
What parts are usually involved with a P0599 code?
The primary component is the electronic thermostat itself, which contains the heater element. Wiring and connectors leading to the thermostat are also common culprits.
How difficult is it to fix P0599 myself?
Fixing P0599 can range from medium to advanced. Inspecting wiring and replacing the thermostat is often manageable for DIYers, but diagnosing complex electrical shorts may require more advanced skills and tools.
Will replacing the thermostat fix P0599?
Yes, if the internal heater element of the thermostat is faulty and causing the high voltage condition, replacing the electronic thermostat assembly will likely fix the P0599 code. However, always check wiring first.