P0599: Thermostat Heater Control Circuit High
What to do
Driveable with little noticeable effect beyond a small loss of fuel economy. Worth repairing at the next service rather than urgently.
What this code means
The thermostat heater circuit is reading high — an open, or a control wire that has found a supply voltage. The mirror of the circuit-low fault, and the last of P05xx's decimal-numbered slots — the block runs on into its hexadecimal ones from P059A.
As with P0597, the practical effect is that the thermostat reverts to purely mechanical behaviour. The engine still warms up and still cools; it simply loses the ability to have its target temperature varied deliberately.
Likely causes
Start here
- Open heater supply circuit · moderate
- Failed thermostat heater element · moderate
- Corroded or loose connector · easy
Less common
- Signal wire shorted to voltage · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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