P0597 – Thermostat heater control system -circuit open

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By Reinier (Contact Me)
Last Updated 2018-02-17
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0597 Thermostat heater control system -circuit open
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Table of Contents

  1. What Does Code P0597 Mean?
  2. Where is the P0597 sensor located?
  3. What are the common causes of code P0597 ?
  4. Get Help with P0597

What Does Code P0597 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 1850F (850C), 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 2300F (1100C), 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 2300F (1100C), 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 1850F (850C), which in turn, improves overall engine performance.

When the PCM determines that driving conditions have once again returned to city driving, it will deactivate the thermostat heater element, which allows the thermostat to revert no normal operation, i.e., using only the wax pellet and return spring to control its operation, thereby again creating the high engine temperature required to reduce emissions during city driving conditions.

From the above it should be obvious that for a mapped thermostat to work properly, its electrical control circuits need to be in perfect working order. Thus, when the PCM detects an open circuit or loss of continuity in the thermostat’s control circuits, it will recognize that it cannot control the thermostat effectively, and it will set code P0597 as a result. Whether or not a warning light is illuminated on the first failure largely depends on the application, and the exact cause of the open circuit.

Where is the P0597 sensor located?

The image above shows a simplified schematic of a mapped thermostat. Note that while all mapped thermostats utilize the same basic components, the actual appearance of these thermostats varies greatly between applications. However, since the introduction of mapped thermostats did not require that engines and cooling systems be re designed or changed in meaningful ways, these thermostats are typically located in the same places where conventional thermostats used to be.

Typically, all thermostats are usually located close to the cylinder head, if not actually in the cylinder head. If a repair manual for the affected application is not available, the easiest way to locate the thermostat is to locate the bottom radiator hose, and to trace this hose back to the engine, where the hose will usually terminate in a plastic or aluminum housing/casing that encloses the thermostat.

Note though that on some applications, the thermostat is located between the engine and the top radiator hose. Nonetheless, a mapped cooling system can be identified by the fact that there will be an electrical connector on the thermostat housing, which also serves to make it easier to locate the actual thermostat in the cooling system.

Be aware though that on some applications, it may be necessary to remove or partially disassemble unrelated components such as the A/C compressors and/or hoses, power steering pump and/or hoses, drive belt(s)/idler pulleys/tensioning devices, and sometimes even the radiator/radiator cooling fan(s) to gain access to the thermostat for the purpose of replacement.

What are the common causes of code P0597 ?

Some common causes of code P0597 could include the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring and or connectors between the thermostat and the PCM
  • Defective thermostat heater element
  • Defective engine coolant sensor(s)
  • Failed or failing PCM. Note that this is a rare event, and the fault must therefore be sought elsewhere before any control module is replaced

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