Engine Speed Signal Incorrect
Engine speed signal is outside the expected range.
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What it means (plain English)
The engine control module (ECM) relies on the engine speed signal from the crankshaft position sensor to determine how fast the engine is running, which affects fuel injection timing and engine performance.
What the computer is actually seeing
The ECM expects a specific frequency signal from the crankshaft position sensor that corresponds to engine RPM; if this frequency is too low or too high, the ECM sets SPN 31.
What a healthy reading looks like
Engine speed signal should correlate to expected RPM values, typically between 600 to 3000 RPM during normal operation.
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 the condition of the crankshaft position sensor and its wiring. Ensure there are no breaks, corrosion, or loose connections.
- 22. Measure the voltage at the crankshaft position sensor while cranking the engine. Expected voltage signal is typically around 0.5 to 4.5 volts. If low or absent, replace the sensor.
- 33. If the sensor is good, use a scan tool to read the actual engine RPM. Compare this value to the expected RPM based on engine speed. If they don't match, check for timing issues or ECM faults.
Common causes
- Faulty crankshaft position sensor
- Damaged wiring or connectors related to the sensor
- Bad engine speed signal due to timing issues
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
No, you should not drive the vehicle as it can lead to poor engine performance and possible stalling.
Will this code come back after clearing it?
If the underlying issue isn't fixed, yes, the fault code will likely return after a short time.
Why do I see SPN 31 with other codes?
It may be accompanied by codes related to engine speed or performance, indicating multiple related issues.
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