P0599: Thermostat Heater Control Circuit High

What is the engine computer reporting with P0599?

The computer detects the thermostat heater control circuit voltage staying higher than expected, pointing to a short to power rather than an open circuit.

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
• Temperature gauge running a little off normal
• Possible overheating tendency on some vehicles
• Reduced fuel economy

What is actually behind P0599?

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.

Wiring short to power is the one to rule out before anything else. This pushes circuit voltage up constantly.

How do you diagnose P0599?

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. Wiring short to power: This pushes circuit voltage up constantly.
• 2. Corroded connector at the thermostat housing: A cheap and common cause to check first.
• 3. Failed heater element: A shorted element can push circuit voltage high.
• 4. Damaged wiring insulation allowing contact with a power source: Chafing near other wiring harnesses can cause this.
• 5. Failing driver circuit inside the computer: Least likely, but possible.

Can you keep driving with P0599?

Keep an eye on the temperature gauge since losing precise electronic control of the thermostat can affect how the engine manages coolant temperature and lead to overheating on some vehicles.

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 P0599?

The main cost is slightly worse fuel economy and emissions, and on some vehicles a real risk of overheating if the thermostat cannot be controlled correctly.

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.