Engine Coolant Temperature Sensor 2 Voltage Above Normal
Indicates that the second engine coolant temperature sensor voltage is too high.
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What it means (plain English)
The computer is reading a higher voltage from the second coolant temperature sensor than it's designed to see, which usually means there's an electrical issue or a sensor problem.
What the computer is actually seeing
The Engine Control Module (ECM) expects a certain range of voltage from the coolant temperature sensor, typically between 0.5V (low temp) to 4.5V (high temp). When the sensor exceeds this range, it triggers the SPN 1878 fault.
What a healthy reading looks like
Normal voltage values from the coolant temperature sensor should be around 1V to 2.5V at coolant temperatures between 50°C (122°F) and 90°C (194°F).
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 wiring and connectors related to coolant temperature sensor 2 for fraying, corrosion, or loose connections. Decision: If damaged, repair or replace as needed; if not, proceed to step 2.
- 22. Test the coolant temperature sensor 2. Use a multimeter to measure the voltage output while the engine is running and compare it to expected values (1-2.5V). Decision: If outside the expected range, replace the sensor; if within range, proceed to step 3.
- 33. Check for any intermittent issues by wiggling the harness while monitoring voltage. Decision: If voltage fluctuates, repair the harness; if stable, move to step 4.
- 44. Clear the fault code and run the vehicle to see if the code returns. Decision: If the code does not return, repairs are complete; if it does, check for ECM updates or internal faults.
Common causes
- Faulty coolant temperature sensor 2
- Wiring issue or short to voltage in the sensor circuit
- ECM error
- Poor ground connection related to the sensor
Typical repair cost
$$
Related codes
Frequently asked questions
Is it safe to drive with SPN 1878 active?
It's generally safe to drive, but keep an eye on engine temperature to avoid overheating.
Will this code come back after fixing the sensor?
If the underlying issue is resolved, the code shouldn't return. Monitor and verify after repairs.
Why does this code appear with SPN 111?
SPN 1878 often pairs with SPN 111 because both relate to coolant temperature sensing, indicating a shared issue in the system.
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