Thermostat Heater Control Circuit/Open


The PCM has detected an abnormal voltage reading from the electronically controlled thermostat heater circuit. Select models require multiple ignition cycles for a service engine soon lamp to be illuminated. A code should be stored on the initial failure but may appear as a pending code until multiple failures are detected.

Code Set Parameters

If the PCM detects a voltage reading from the electronic thermostat assembly that is not within a predetermined manufacturer’s reference degree, after the engine has run for a set period of time, a code will be stored and a service engine soon lamp may be illuminated (see note above).


Depending upon the position of the electronically controlled thermostat at the time of failure, symptoms may vary greatly. Should the thermostat fail after the engine has reached normal operating temperature, the chances of any drivability symptoms being noticed are minimal. If the unit fails when the engine is cold, overheating issues could present themselves. If the thermostat fails when the engine is very warm, the amount of time required for the engine to reach normal operating temperature (especially in cold climates) could be extended. Of course, a code should be stored on the first failure and a malfunction indicator lamp will be illuminated after consecutive failures (usually it takes 3).

Common Causes

The most common cause of this code being stored comes from defects in system circuitry. Loose, broken, or corroded wiring and electrical connectors are common and should be investigated first. Other possible causes include a defective electronically operated thermostat, the control housing (complete with computerized circuitry) in which the electronically controlled thermostat is often integrated could also be defective, the PCM could be faulty but this is a rare occurrence. A low engine coolant condition may also cause this code to be stored. If the engine overheats or if the cooling fan operates prematurely, this code may be stored. If there are other codes related to engine temperature or drivability are stored prior to this code being stored, diagnose and repair those before attempting to diagnose this code.

Common Misdiagnosis

The entire electronic thermostat is frequently replaced when a problem with system circuitry is the real culprit. Technicians report that corrosion caused by leaking engine coolant may cause wiring and connectors to deteriorate and eventually fail.


  • The electronically controlled thermostat, as used in light automotive applications, is usually part of the Bosch family of automotive components
  • The integrated thermostat and electronic control unit is called a Motronic control unit
  • This electronically controlled thermostat uses inputs from various temperature and engine load sensors to calculate and adjust the amount of engine coolant flow allowed through the engine passages
  • In theory, this type of system can allow the engine to operate more efficiently by regulating overall engine temperature during different driving conditions
  • For instance, as engine RPMs increase, so does coolant flow through the radiator and engine passages
  • This increased coolant flow causes a decrease in engine temperature, thus increasing the potential for engine performance output and allegedly decreasing fuel efficiency
  • The electronically controlled thermostat is designed to decrease coolant flow at higher RPMs and promote increased fuel efficiency (albeit at the expense of increased engine performance)
  • This is only a small fraction of the vast number of functions for which the electronically controlled thermostat is responsible. Begin your diagnosis by removing the radiator (or pressurized reservoir) cap from the cold engine cooling system and making sure that the radiator is filled to the correct limit
  • If the coolant is excessively low, fill to the correct level and perform a system pressure test to find the coolant leak
  • Repair leaks as required and retest the system
  • Several tools will be instrumental in successfully diagnosing this code
  • A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in performing a successful diagnosis
  • An infrared thermometer with a laser pointer will also be helpful
  • A coolant system pressure tester (with suitable adapters) may also be helpful if a low coolant level is detected. Continue with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • Continue by clearing the code and operating the vehicle to see if it returns
  • This will help to determine whether or not the malfunction is intermittent
  • After the codes are cleared, test drive the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
  • If no obvious problems are detected in system circuitry or electrical connectors, disconnect the electrical connector from the thermostat control unit and test it for voltage and ground signals
  • Repair open or shorted circuits as needed and retest the system
  • If your findings line up with the manufacturer’s referenced voltage amounts, reconnect the electronic thermostat and test system inputs and outputs
  • Compare your actual findings to manufacturer’s specifications and repair open or shorted circuits or components as necessary
  • Use care when testing the Motronic unit and avoid testing the unit for resistance when voltage is present. If the condition is yet to be rectified, disconnect the electrical connector from the electronically controlled thermostat and perform a resistance test
  • Compare the ohms of resistance, related to the respective circuits (for the varying degrees of temperature) with the manufacturer’s specifications
  • If resistance levels fail to coincide with the manufacturer’s referenced amounts, replace the thermostat, refill the radiator with coolant, and retest the system. If the thermostat proves to be in proper working order, disconnect the electrical connector from the thermostat control unit and the PCM
  • Perform a continuity test using the digital volt/ohmmeter
  • Compare your actual findings with the manufacturer’s specifications and repair or replace wiring circuits or connectors as required