Engine Coolant Temperature Sensor 1 High
The engine coolant temperature sensor signal is too high for normal operation.
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What it means (plain English)
The engine's computer is reading that the coolant is hotter than it should be, indicating possible overheating or sensor issues.
What the computer is actually seeing
The engine control module (ECM) expects a voltage from the coolant temperature sensor between 0.5V (cold) and 4.5V (hot). When the voltage exceeds 4.5V, it triggers SPN 6704.
What a healthy reading looks like
An expected voltage range from the sensor is typically 0.5V to 4.5V, corresponding to coolant temperatures from about 0°C (32°F) to 100°C (212°F). Anything above 4.5V is considered too high.
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. First, check the coolant level. Ensure it’s full and not leaking.
- 22. Inspect the wiring and connector to the coolant temperature sensor for corrosion or damage.
- 33. Measure the voltage at the sensor connector while the engine is at operating temperature. It should be between 0.5V and 4.5V.
- 44. If the voltage is above 4.5V, replace the coolant temperature sensor and retest.
- 55. If the sensor voltage appears normal, test for short circuits in the wiring harness between the ECM and sensor.
Common causes
- Faulty coolant temperature sensor
- Wiring harness issue (short circuit)
- ECM malfunction
- Low coolant level causing overheating
- Corroded or loose connector at the sensor
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code active?
It's best to fix it soon, as an overheating engine can lead to serious damage.
Will this code come back after fixing?
If the underlying issue is resolved and the sensor is replaced, it shouldn't come back.
What does this code have to do with other sensors?
If the coolant temperature sensor reading is incorrect, it can affect other systems, such as fuel injection timing and emissions.
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