FMI 18 - Transmission Fluid Temperature Sensor
Transmission fluid temperature sensor signal is out of range.
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What it means (plain English)
The computer is getting a reading from the transmission fluid temperature sensor that is too high or too low, which could cause transmission issues or overheating.
What the computer is actually seeing
The engine control module (ECM) expects the transmission fluid temperature to be between 80°F and 230°F while operating. Values outside this range trigger the fault code, indicating a potential issue with the sensor or transmission system.
What a healthy reading looks like
Normal transmission fluid temperatures should range from 80°F to 230°F (27°C to 110°C).
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. Inspect the transmission fluid level. Ensure it is at the correct level and the fluid is not burnt or contaminated.
- 22. Check wiring and connector for the transmission fluid temperature sensor. Look for frayed wires or loose connections.
- 33. Using a multimeter, test the resistance of the transmission fluid temperature sensor at different temperatures. Compare readings to expected values (typically 3-5 ohms at 80°F and around 100-120 ohms at 212°F).
- 44. If sensor readings are out of range, replace the temperature sensor and recheck for codes.
- 55. If the code returns, inspect the wiring harness and connectors for damage or short circuits.
Common causes
- Faulty transmission fluid temperature sensor
- Wiring issues or short circuits
- Fluid contamination or incorrect fluid level
Typical repair cost
$$ - $$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
It's generally safe to drive, but avoid heavy loads or towing until fixed.
Will this code come back after clearing?
It may if the underlying issue isn’t resolved.
Why does this code often pair with SPN 1764?
SPN 1764 indicates pressure issues that can also relate to poor fluid temperature readings.
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