P0597: Thermostat Heater Control Circuit/Open

What P0597 tells you, and what it does not

Many newer engines use an electronically heated thermostat that the computer can open earlier than a plain mechanical thermostat would, and this code means the computer found an open circuit in that heater element or its wiring.

The code sits in the electronic thermostat control area and is reported by the comprehensive component monitor. It names the circuit or the condition where the computer saw a reading outside its expected range. It does not name a failed part, and several genuinely different faults set this same code, which is the whole reason the causes below are ordered rather than listed.

What people usually notice

• Check engine light with no noticeable drivability change
• Slightly different warm up behavior
• Temperature gauge running a little higher or lower than normal
• Reduced fuel economy

What causes P0597?

The causes below are ordered by how often they turn out to be the answer, cheapest and most common first. That order matters more than the list itself: the expensive component named in the code text is very rarely the first thing to replace, and working down from the top is what stops a electronic thermostat control fault becoming an expensive guess.

Corroded connector at the thermostat housing is the one to rule out before anything else. The most common and cheapest cause to check first.

Working out which cause it is

Start with freeze frame. Check the coolant temperature stored when the code set and compare it to normal operating temperature. That snapshot is taken at the instant the code set, and it is the single most useful piece of evidence you have, which is also why clearing the code before reading it is such a costly habit.

This code is diagnosed by watching values change rather than by reading the number again, so the tool has to stream live data and hold freeze frame. Live data on coolant temperature versus commanded thermostat heater state shows whether the element is responding.

The order worth working in

• 1. Corroded connector at the thermostat housing: The most common and cheapest cause to check first.
• 2. Wiring chafed near the engine block: Heat and vibration in this area wear insulation over time.
• 3. Failed heater element inside the thermostat: The element itself can burn out.
• 4. Blown fuse in the circuit: A lost supply voltage reads as an open circuit.
• 5. Failing driver circuit inside the computer: Least likely, but the output stage controlling the heater can fail.

What P0597 means for driving the car

Keep an eye on the temperature gauge since the thermostat will likely behave like a standard mechanical unit and precise coolant temperature control is lost.

This is a severity 2 of 4 code. It will not usually strand you, but it will fail an emissions test while it is present, and the underlying fault rarely gets cheaper by being left alone.

What does it cost to ignore P0597?

The main cost is slightly worse fuel economy and emissions from imprecise coolant temperature control, it is not an urgent mechanical risk by itself.

There is also the diagnostic cost of waiting. Freeze frame data, pending code history and the readiness monitor state all describe the fault as it was when it set, and all of it is lost the moment somebody clears the code to make the lamp go out.