OBD-II trouble code
P0599: Thermostat Heater Control Circuit High
The thermostat heater circuit read high when it should have been pulled low. On this particular circuit the honest advice is not to over-read the difference between the high code and the open code — one resistance measurement at the connector tells you more than either code number does.
Quick facts
- System
- Powertrain
- Category
- Cooling / Thermostat
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $0 – $700
- DIY difficulty
- Intermediate DIY
What does P0599 mean?
The map-controlled thermostat carries a small resistive heater inside its wax pellet, and the engine control module energises it to force the thermostat open earlier than its mechanical setpoint under heavy load. P0599 is set when the module switched that heater on and the voltage at its output did not fall the way a completed circuit requires.
The useful thing to understand here is why that description overlaps so heavily with P0597. The heater is driven by a low-side switch: battery voltage is present at one end of the element permanently, and the module completes the circuit by connecting the other end to ground. When the module turns the driver on, it monitors its own output pin. If the element and its wiring are intact, that pin is pulled close to ground and the module sees a low voltage. If anything in the path is broken — an open heater winding, a parted wire, a half-latched connector, a corroded terminal — the pin stays sitting at battery voltage through the element's own feed, and the module reports a high state.
The consequence is that on this circuit, an open circuit and a high circuit are frequently the same physical fault reported under two different code numbers, and which one a given vehicle sets is a calibration decision rather than a diagnostic distinction. Some manufacturers set P0597 for any loss of continuity and reserve P0599 for a genuine short to a voltage source; others set P0599 for the whole family of open faults. Chasing the semantics of the number is wasted effort. What actually separates the possibilities is one measurement, described below.
Unplug the thermostat and measure across its two terminals. A heated thermostat element typically reads somewhere in the region of a few ohms to a couple of dozen ohms cold, with the exact figure varying by application, so the specification matters more than any general number. What does not vary is the interpretation. An infinite reading means the element is open and the thermostat is the fault. A reading close to zero means the element is shorted and belongs to P0598 territory. A reading in the plausible band means the element is fine and the break is in the wiring, the connector, or the module. That single check collapses three code numbers into one answer and is the reason this page recommends the meter over the code list.
The genuine short-to-voltage case does exist and is worth ruling in or out where the measurement exonerates the thermostat. It happens where the thermostat harness branch runs alongside a permanently live feed and chafes against it, so the module's output is being held up by an external source it cannot overcome. That fault behaves differently from an open: the circuit reads high even with the thermostat unplugged and the module disconnected, which is a decisive and quick test.
As with the rest of this block, the fault has no effect on whether the engine will overheat. The thermostat still opens mechanically at its rated temperature. What is lost is the software's ability to drop coolant temperature deliberately when the engine is being worked hard.
Common causes
- Open heater winding inside the thermostat, which many calibrations report as a high state rather than as an open circuit
- Half-latched or partly unseated connector at the thermostat leaving one terminal not making contact
- Corroded terminal in the thermostat connector with enough resistance to prevent the output being pulled low
- Broken conductor inside intact insulation on the short harness branch, typically at a heat-cycled crimp
- Short to battery voltage from harness chafe against a permanently live feed
- Failed low-side driver in the engine control module unable to pull its own output down
- Thermostat replaced with a non-heated part, leaving the circuit permanently incomplete
- Connector left unplugged after coolant or timing work
Symptoms
- Check engine light on with P0599 stored
- No change to the temperature gauge and no overheating
- Coolant temperature holding at the higher part-load setpoint under heavy load
- Minor loss of power on calibrations that compensate for the lost cool-down by retarding ignition timing
- Small reduction in fuel economy
- Cooling fans running earlier or longer as the module compensates by other means
- In many cases the warning light is the only symptom
Diagnostic steps
- 1.Unplug the thermostat connector and measure resistance across its two terminals. This single reading separates an open element, a shorted element, and a healthy element with a wiring fault, and it is worth more than the code number.
- 2.Compare the measured value against the specification for the application rather than a general rule, since heated thermostat elements vary considerably between manufacturers.
- 3.If the element measures within specification, check whether the circuit still reads high with both the thermostat and the module disconnected. A high reading in that state proves a short to a voltage source rather than an open.
- 4.Inspect the connector for a terminal that is only partly latched. A half-seated connector looks and feels fitted and is a common cause of an intermittent version of this fault.
- 5.Pull gently on each wire at the back of the connector. A conductor that has parted inside undamaged insulation at a heat-cycled crimp is invisible until it is loaded.
- 6.Check continuity of both circuit wires from the thermostat connector to the module connector, wiggling the harness while watching the meter.
- 7.Confirm that the fitted thermostat is the heated type specified for the vehicle. A non-heated replacement leaves the circuit permanently open and produces this code indefinitely.
- 8.Inspect for coolant residue around the housing, because a wet connector can produce erratic readings that alternate between this code and its low-side sibling.
- 9.After repair, command the heater on with a scan tool where the function exists and confirm the output pulls low and the current draw is sensible.
Repair cost
$0 – $700
A connector that was left unplugged or is only half-latched costs nothing to correct, and a terminal or crimp repair is 0.3-1 hour. Where the element itself has failed, the thermostat is $50-$250 on most applications and higher where it is supplied only as an integrated plastic housing assembly including the coolant temperature sensor, which is normal on the European engines that use this design. Labour is 0.8-2.5 hours plus coolant and a bleed. Because the heater element cannot be replaced separately from the wax pellet, a break worth a few cents forces the whole part — which is exactly why the resistance measurement is worth taking before the part is ordered.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with thermostat replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.