What does it mean?
This is a generic powertrain diagnostic trouble code (DTC), meaning it covers all makes/models from 1996 onward. However, specific troubleshooting steps vary by vehicle.
Codes P0597, P0598, and P0599 all refer to an electronically controlled engine thermostat. These are generic codes, meaning they apply to all vehicles using this type of thermostat control, though only a small number of manufacturers use such systems. Therefore, this trouble code article is essentially identical for all three codes.
Diagnostic code P0598 indicates a failure in this system where the control voltage is low. Mostly, vehicles manufactured in Germany such as BMW, Mercedes, Audi, Mini, Volkswagen, Opel, and Jaguar use this system.
The electronically controlled thermostat consists of various temperature and load sensors, a computer program, and the control unit with an integrated thermostat.
Concept
By electronically controlling coolant flow or engine temperature, partial throttle increases fuel economy and reduces emissions, while reducing temperature under load increases power.
Symptoms
Symptoms vary depending on the thermostat’s position at the time of failure; however, there is unlikely to be a noticeable difference in vehicle operation.
The check engine light comes on and one of the above codes is set
The temperature gauge may read abnormally high if the thermostat failed in a partially closed position; conversely, it will read lower than normal if the thermostat failed in a fully open position
Potential Causes
Experience shows that the issue is usually a loose or corroded electrical connector or the electric thermostat itself is at fault. Sometimes the Motronic computer (engine management) fails, but this is the least likely cause.
A loose or corroded connector on the thermostat
A coolant leak will trigger the code
The thermostat itself has failed
A short circuit or open circuit occurred in the wiring harness between the computer and the thermostat
The Motronic computer has failed
Diagnostic and Repair Procedures
Remove and inspect the electrical connector. Eliminate any corrosion using baking soda or by scraping. Apply electrical grease and confirm a tight connection.
Inspect the coolant level in the radiator. Low coolant levels will set a code by causing the electronic thermostat to overheat.
Remove the electrical connector and check the resistance values on the thermostat. For this procedure, you will need a service manual or the necessary information available online.
This information should include pin identification and location, wire colors, pin voltage values at specific temperatures, and resistances. These values and the diagnostic flow vary by manufacturer and engine size.
You will also need an infrared temperature probe and a volt/ohmmeter.
Determine the engine temperature with the temperature probe
Start the engine and test the voltage on the Motronic side of the harness as per instructions. If it is within limits, continue testing. If there was no voltage or it was out of range, replace the Motronic unit
Compare the resistance on the thermostat side of the harness to the thermostat. If the resistance is out of range, replace the thermostat unit
If tools are not readily available, take the vehicle to a reputable shop that has access to the necessary information. This is a simple diagnosis with the right tools