Thermostat Heater Control Circuit/Open
What it means
Some engines use an electronically controlled thermostat with a small heater that the engine computer switches on to open it early and lower engine temperature under load. The computer has found this heater circuit open or not responding, so it can no longer control temperature this way.
Related lights
Dashboard lights that may come on with this code.
Symptoms4
- Check engine light on.
- Engine temperature slightly higher than usual when driving under load or in heat.
- Slight increase in fuel use or change in engine performance.
- Often no clear symptoms other than the light.
Possible causes5
- Failed heating element inside the thermostat.
- Connector corroded by coolant leaking into it.
- Open heater wiring.
- Blown fuse in the heater supply circuit.
- Fault in the driver circuit inside the engine computer (rare).
How to diagnose5
- Inspect the connector for coolant or corrosion.
- Command the heater on with a scan tool if possible and watch the response.
- Measure the heating element resistance and compare it with the manufacturer specification; infinite resistance means an open.
- Check the fuse and the supply voltage at the connector.
- Check continuity of the wire between the connector and the computer.
How to fix5
- Replace the fuse after confirming there is no short.
- Clean or replace the connector and fix any coolant leak.
- Repair broken wiring.
- Replace the complete electronic thermostat, since the heater is part of it on most vehicles.
- Clear the codes and confirm engine temperature stays in the normal range.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
In the Saudi market: SAR 100–1,500 (≈ USD 27–400)
Our prices come from the Saudi market, converted at a fixed rate (SAR 3.75 per USD). Prices in your country may differ a lot, so we show the cost level.
The cost is low for a connector or wire fault, and rises when the electronic thermostat is replaced, since it costs more than a standard one and the coolant must be drained.
Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026
Frequently asked questions
Does this code cause overheating?
It does not usually cause boiling, because the thermostat still works in the conventional way, but temperature may run slightly higher under load. Watch the temperature gauge.
Can the heater be replaced on its own?
On most vehicles no, because the heater is built into the thermostat, so the whole unit is replaced.
Was this explanation helpful?
Tap the stars to rate; you can change it later.
What was wrong? (optional)
We never store your IP address, and your comment is not published. Details: Privacy policy · Terms and conditions
Report a mistake
Spotted something inaccurate? Tell us and we will review and correct it.
Report received — thank you
This is your private tracking link. It shows the status and our reply as soon as it is posted.
The link is saved in this browser only.
