Insufficient Engine Speed Decrease During Self Test
During self-test, your engine should rev down quickly when the ECU commands it, but it's not slowing down enough. Think of it like pressing the brakes on a car and the wheels not responding as expected.
During self-test mode, the ECU commands the engine speed to decrease and monitors RPM drop rate using the crankshaft position sensor. If engine speed doesn't decrease within the expected time window, the ECU flags insufficient deceleration. This typically involves fuel cutoff or ignition timing adjustments.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| RPM decrease rate during self-test | Minimum 200-400 RPM drop in specified time window | RPM drop below threshold; insufficient deceleration detected |
| Self-test engine speed window | Engine returns to idle range (600-800 RPM) within 2-3 seconds | Engine remains above idle threshold longer than allowed |
Code P1731 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, P1731 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.