What does it mean?
OBD II fault code P0597 is a generic code that is defined as “Thermostat heater control system -circuit open”, and is set when the PCM (Powertrain Control Module) detects an open circuit (loss of continuity) in the heater control circuit of an electrically assisted thermostat, also known as a “mapped” thermostat. Note that while this technology is in widespread use in Europe, it has become increasingly more common in the North American and other markets in recent years.
While conventional thermostats that use an expansion material and a spring to open and close a valve in order to regulate the engine temperature have proven to be reasonably reliable and efficient over many years, these thermostats are limited to two basic modes of operation that are both linked to the thermostat’s design, or control range.
Essentially, a fully functional conventional thermostat can only be open to allow coolant to circulate through the radiator to shed heat, or, it can closed to prevent coolant from circulating freely through the engine to allow the engine to warm up quickly.
In practice, there is no in-between stage: if the cooling system is fully functional, the thermostat will open at its minimum design temperature, and from that point on, the thermostat will maintain the coolant temperature at, or close, to the maximum allowable temperature until the engine is shut off, and circulation of the coolant ceases.
However, since exhaust emissions are closely linked to the engine’s temperature, increasingly stringent emissions requirements have forced engine designers to devise new ways to control engine temperatures in order to limit, or reduce exhaust emissions over as wide a range of engine temperatures as possible.
For instance, at engine temperatures of lower than about 185 0 F (85 0 C), more engine power can be produced since at these temperatures, engine knock is reduced while ignition timing can be optimized, and particularly under high engine load conditions. However, at low engine temperatures combustion is less efficient, which causes both higher fuel consumption and emissions.
On the other hand, at engine temperatures of up to about 230 0 F (110 0 C), combustion is vastly more efficient, and particularly under light engine load conditions such as those experienced during low speed city driving at relatively small throttle openings. Under these conditions, fuel economy is increased, and emissions are drastically reduced.
Therefore, by adapting the engine temperature to driving conditions, emissions can be reduced significantly without sacrificing either fuel economy or engine power, and this is exactly what a mapped, or electrically assisted thermostat does.
In terms of operation, a mapped thermostat still uses a wax pellet and a spring to control its operation, but the wax pellet is augmented by a heater element (controlled by the PCM) to create a means of controlling the operating characteristics of the thermostat based on driving conditions.
In practice, the PCM uses input data such as engine speed, throttle position, intake air and engine coolant temperatures (and others) the PCM is able to determine operating conditions, and hence a suitable thermostat setting that will bring the engine temperature into range where emissions can be reduced without sacrificing fuel economy or engine efficiency.
Thus, when the PCM determines that a low speed, low engine load condition such as city driving is present, it will not activate the thermostat heater, which allows the thermostat to open at its maximum design temperature (typically about 230 0 F (110 0 C), since this shortens engine warm-up times, improves fuel economy, and reduces emissions.
However, should operating conditions change and a lower engine temperature is required for, for instance, high speed highway driving at relatively large throttle openings, the PCM will activate the thermostat heater to open the thermostat sooner, which reduces the engine temperature to about 185 0 F (85