Thermostat - Coolant Temperature below Normal Operating Range
Indicates that the engine coolant thermostat is stuck open.
Heads up: this code wasn't in our hand-written library yet, so DiagCoach AI built this page on the fly. Double-check critical specs against service info before condemning parts.
What it means (plain English)
The thermostat is supposed to close to keep the engine cool, but it's stuck open, so the engine isn't warming up properly.
What the computer is actually seeing
The Engine Control Module (ECM) sees an engine coolant temperature below the expected range, typically under 185°F (85°C), when the engine should be warming up.
What a healthy reading looks like
The engine coolant temperature should be 185°F to 220°F (85°C to 104°C) when at operating temp, not reaching that indicates a problem.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 1Check the engine coolant level — should be between Min and Max on the reservoir.
- 2Inspect the thermostat housing for leaks or damage.
- 3With the engine at operating temperature (about 190°F), check if the upper radiator hose is hot. If it's not, the thermostat may be stuck open.
- 4Use a scan tool and check the coolant temperature reading; it should rise steadily if the thermostat is functioning properly.
- 5If the coolant sensor shows a reading below 185°F with the engine at temp, replace the sensor; if the hose is cool, replace the thermostat.
Common causes
- Stuck open thermostat
- Low coolant level
- Faulty coolant temperature sensor
- Wiring issues to the thermostat or sensor
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
It's drivable, but the engine may not run efficiently, leading to higher emissions and degraded performance.
Will this code come back after clearing?
If the problem isn't fixed, expect this code to return.
Why does this code sometimes pair with SPN 1929?
Both codes relate to temperature control; SPN 1929 indicates the engine is running cold as well.
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