Excessive Heat in Engine Coolant
Indicates the engine's coolant temperature is too high.
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What it means (plain English)
The engine computer sees that the coolant temperature has risen above the normal range, which should be between 180°F and 220°F (82°C to 104°C) during operation.
What the computer is actually seeing
The engine control module (ECM) monitors coolant temperature via the temperature sensor. If the sensor reports temperatures exceeding 230°F (110°C), it triggers the fault code SPN 1689.
What a healthy reading looks like
Coolant temperature usually should not exceed 220°F (104°C) during normal operation. If readings show above 230°F (110°C), it indicates overheating.
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. Visually inspect the coolant level in the overflow tank and radiator. Is it low? If yes, refill and check for leaks.
- 22. Check for any visible leaks around hoses and connections. Any signs of leaks? If yes, repair them before proceeding.
- 33. Test the coolant temperature sensor. Unplug the sensor and measure the resistance; it should be between 2,000 - 3,000 ohms at 70°F (21°C). Is it within range? If no, replace the sensor.
- 44. Verify operation of the thermostat by allowing the engine to reach operating temperature. Does the upper radiator hose get hot? If not, thermostat may be stuck closed.
- 55. Inspect the water pump for proper function, listening for leaks or unusual noises during operation. Is it working properly? If no, replace the water pump.
Common causes
- Low coolant level due to leaks
- Faulty coolant temperature sensor
- Thermostat stuck closed
- Failed water pump
- Clogged radiator or cooling passages
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
No, it can cause engine damage if you ignore it. Overheating can lead to a blown head gasket or cracked engine block.
Will this code come back after clearing it?
Yes, if the underlying issue isn't fixed, the code will reappear.
Why does this code pair with SPN 1688?
Both codes relate to engine temperature, indicating possible simultaneous overheating issues.
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