Engine Coolant Temperature (ECT) Sensor
Coolant temperature sensor reading is out of expected range.
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What it means (plain English)
The engine's computer monitors the temperature of the coolant to prevent overheating. If the coolant is too hot or cold, the sensor sends a signal that's way out here, making the computer think something is wrong.
What the computer is actually seeing
An abnormal voltage signal from the ECT sensor, typically between 0.5V (cold) and 4.5V (hot) in normal operation. If the reading falls below 0.2V or above 4.8V, it triggers the fault code.
What a healthy reading looks like
ECT should read between 140°F (60°C) to 230°F (110°C) in normal operation, translating to 1V-3.5V signal range.
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 coolant level; ensure it is above the minimum level in the reservoir. If low, refill and retest.
- 22. Inspect wiring and connectors at the ECT sensor for damage or corrosion. Repair as necessary.
- 33. Measure ECT sensor resistance with a multimeter. It should decrease when in heat, typically dropping from ~10K ohms at 32°F to ~200 ohms at 212°F.
- 44. If the sensor and wiring are good, check the engine for overheating conditions, such as a malfunctioning thermostat or low coolant flow.
Common causes
- Faulty ECT sensor
- Wiring issues or damaged connectors
- Coolant level low or inconsistent
- Engine overheating
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with SPN 599 active?
It's moderate severity, so it's best to fix soon to prevent engine damage from overheating.
Will this fault code come back after a reset?
If the underlying issue is fixed, it should not return.
Why does SPN 599 sometimes show up with SPN 110?
SPN 110 indicates a potential bad coolant temperature reading alongside the ECT signal being out of range.
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