Engine Coolant Temperature
The engine coolant temperature is too high or low, indicating a potential issue with cooling system performance.
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What it means (plain English)
The coolant temperature sensor is reading a temperature that's outside of normal operating conditions, either too hot (over 245°F / 118°C) or too cold (below 32°F / 0°C), which means the engine might not be cooling properly or there could be a sensor failure.
What the computer is actually seeing
The engine control unit (ECU) expects a signal from the coolant temperature sensor that corresponds to a temperature range of about 160°F (71°C) to 240°F (116°C). Outside this range, it sets SPN 40.
What a healthy reading looks like
Normal coolant temperature should be between 160°F and 220°F (71°C to 104°C), with specific thresholds being 245°F (118°C) as a max temperature before warning.
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.
- 11. Check the engine coolant level; top off if low, and inspect for leaks.
- 22. Inspect the wiring and connectors of the coolant temperature sensor for damage or corrosion.
- 33. Using a scan tool, monitor the coolant temperature reading; compare it with actual coolant temp using an infrared thermometer. They should match closely.
- 44. If readings are normal but the code persists, inspect or replace the coolant temperature sensor.
- 55. If the sensor is good, check for a sticking thermostat by observing engine temperature changes; replace if faulty.
Common causes
- Faulty coolant temperature sensor
- Wiring issues or bad connections in the sensor circuit
- Low coolant level or leaks in the cooling system
- Faulty engine thermostat
- Overheating due to radiator blockage or failed water pump
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
It may be drivable, but monitor the temperature gauge closely. If it overheats, it can cause severe engine damage.
Will this code come back after clearing?
If the underlying issue is not resolved (like a bad sensor or low coolant), the code will likely return.
Why does this code pair with SPN 41?
SPN 41 indicates a related issue with temperature control, often seen with malfunctions in the same system.
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