Auxillary Speed Sensor Fault
Your engine's auxiliary speed sensor isn't communicating properly with the computer, like a speedometer that's giving wrong readings. The ECU can't accurately monitor engine speed variations, which affects idle control and fuel injection timing.
The ECU monitors voltage signals from the auxiliary speed sensor to detect engine RPM fluctuations and load conditions. The sensor typically generates a frequency or analog signal that the ECM compares against expected ranges. If the signal is absent, intermittent, or out of acceptable thresholds, the fault is triggered.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Signal Voltage | 0.5–4.5 volts (frequency-dependent) | Below 0.2V or above 4.8V continuously |
| Signal Frequency/Stability | Consistent pulse within operating range | Missing, erratic, or no change over 2+ seconds |
Code P1508 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P1508 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.