P0599: Thermostat heater control circuit voltage too high

Thermostat heater control circuit high

Severity: Severity not verified — if the car runs badly, have it inspected

The sources define the code and report a hard-running cooling fan and possible overheating, but do not rate its urgency.

It gets more serious if:

Inspect soon: Get it inspected soon if the temperature gauge climbs or the cooling fan runs hard all the time.

Source: how we rate severity

P0599 in short

P0599 means the computer sees too much voltage in the circuit that heats an electronically controlled thermostat. It applies only to engines with that type of thermostat. A burnt-out heater element is a common cause. Owners also report a fan running hard, so check the cooling system soon.

Some engines use a thermostat with a small heater that lets the computer open it early, independent of coolant temperature. The computer watches the heater circuit.

P0599 is set when that circuit reads high. TroubleCodes.net lists a burnt heater element as a very common cause, along with coolant temperature sensors, wiring and the thermostat.

Owners of a 2012 Chevrolet Cruze and a 2011 Mini Cooper Countryman report the code with a hard-running cooling fan or concern about overheating.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

Cause 1: Electronically heated thermostat (heater element)
Common general cause
Why it ranks here
TroubleCodes.net calls a burnt thermostat heater element very common for this code.
What would change this
Rises if the heater resistance is out of spec. Falls if it is in spec.
How to test the thermostat
Cause 2: Wiring or connector fault
Common general cause
Why it ranks here
The same page lists shorted or open circuits and connectors.
What would change this
Rises if damage or corrosion is found. Falls if the circuit tests good.
Cause 3: Coolant temperature sensor fault
Possible — needs testing
Why it ranks here
The page lists defective primary or secondary coolant temperature sensors; the evidence is weaker than for the thermostat itself.
What would change this
Rises if the sensor reading is implausible. Falls if it tracks real temperature.
How to test the engine coolant temperature (ect) sensor

What to test first

Start at step 1. Each result tells you what it means and where to go next.

Step 1: With the engine cold, check the coolant level and read all codes, including any coolant sensor codes.
Low riskTool: OBD-II scan tool
- IfCoolant is low or other coolant codes are stored
  It meansAddress those first.
  Go to step 2
- IfCoolant is full and only P0599 is stored
  It meansCheck the thermostat circuit.
  Go to step 2
Step 2: Have the thermostat heater circuit tested for voltage, ground and element resistance.
Best left to a shopTool: scan tool, multimeter
Warning:Do not open the cooling system while hot; hot coolant under pressure can scald.
- IfHeater element is open or out of spec
  It meansReplace the thermostat.
  Points to: Electronically heated thermostat (heater element)
- IfCircuit is open or shorted
  It meansRepair the wiring.
  Points to: Wiring or connector fault
- IfCircuit and heater are good
  It meansCheck the coolant temperature sensors.
  Points to: Coolant temperature sensor fault

Symptoms

Cooling fan runs hard
Poor heater output
Engine runs hot Gets more serious: stop and have it checked.

How to fix

Step 1: With the engine cold, check that the coolant level is correct.

Parts and cost context

Thermostat:Parts and labour vary; get a quote.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Chevrolet

2012 · Cruze
Supported by vehicle data
One 2012 Cruze complaint reports P0599 read as the thermostat control circuit, with a hard-running fan.

No curated relationships for P0599 yet.

All Engine cooling (thermostat) codes

P0599 questions

Does every car use this circuit?

No. It applies only to engines with an electronically heated thermostat.

Sources and review

P0599 trouble code definition — TroubleCodes.net
ODI 10882251: 2012 Chevrolet Cruze, P0599 thermostat control circuit — NHTSA Office of Defects Investigation (consumer complaint)
ODI 10971166: 2011 Mini Cooper Countryman, P0599 — NHTSA Office of Defects Investigation (consumer complaint)

Last reviewed: October 16, 2026
Written by: Saurabh K.
Editorial review: Pending
Technician review: Technician review pending

This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error

Severity: Severity not verified — if the car runs badly, have it inspected

Most likely causes

Cause 1: Electronically heated thermostat (heater element)

Cause 2: Wiring or connector fault

Cause 3: Coolant temperature sensor fault

What to test first

Step 1: With the engine cold, check the coolant level and read all codes, including any coolant sensor codes.

Step 2: Have the thermostat heater circuit tested for voltage, ground and element resistance.