A P0501 code on your 2021 Honda Civic indicates the ECM has detected that vehicle speed sensor readings are outside expected parameters. Unlike a complete malfunction, this suggests the sensor is working but providing questionable data.
Range/Performance Issues
P0501 is a rationality code—the sensor signal exists but doesn't make sense compared to other data. The ECM may compare speed sensor data to transmission input/output speed ratios, wheel speed sensors, or expected acceleration patterns to identify discrepancies.
Civic Speed Sensing
The 2021 Civic uses electronic speed sensing integrated with the CVT transmission and ABS system. Honda's systems cross-reference multiple speed inputs for accuracy. When one sensor disagrees significantly with others, P0501 sets.
Causes of Range/Performance Faults
A degrading speed sensor provides inaccurate readings. A damaged tone ring creates irregular pulses. Debris or metal contamination near the sensor affects readings. Incorrect tire size affects expected speed-to-RPM ratios. Internal transmission issues can affect output speed sensor readings.
Diagnostic Approach
Use Honda HDS to compare speed readings from different sources. Check wheel speed sensors versus transmission speed sensor. Inspect the tone ring for damage or missing teeth. Verify tire sizes are correct and match. Clean the speed sensor and mounting area of any debris.