Citroen C4 - Lack of engine power | HELLA

If the above vehicle loses power, and simultaneously the air conditioning fails and the malfunction indicator lamp lights up, you should consider a faulty coolant thermostat as a possible cause.

In this situation the engine will usually be running in emergency mode. The following error codes will be recorded in the memory of the engine control module: P0171, synonymous with ‘Bank1 mixture too lean’ and P0597, synonymous with ‘Open circuit in thermostat and heating regulation’.

Since emergency mode has evidently been activated there is a tendency to first follow up the error P0171 and consider the mixture regulation, but in this case this message is in all probability merely a consequence of the real problem. If both these errors are recorded simultaneously you should look for the problem in a defective electronic thermostat. These thermostats have electrical heating resistors, and can be specifically controlled to accelerate the temperature rise of the motor, this improving its exhaust performance. If the relevant signal is not received, for instance because the heating resistor has burned out, the motor control module will initiate emergency operation in order to prevent the damage that might be caused by overheating. This thermostat, which is located to the left of the cylinder head, is available only as a complete unit, including housing.