Idle Signal Circuit Malfunction
Your engine's idle speed control isn't communicating properly with the computer—think of it like a broken thermostat that can't tell the furnace to adjust the heat. The ECU can't maintain smooth idling because the idle air control signal is missing or erratic.
The ECM monitors the idle air control valve (IACV) or electronic throttle control signal to maintain target RPM during idle. It expects a stable voltage signal within a defined range; deviation triggers this fault when the signal is absent, erratic, or out of specification for a set duration.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Idle Air Control Voltage | 0.5–4.5V (varies by vehicle) | Out of range or no signal detected for >2 seconds |
| Idle RPM Deviation | ±50 RPM of target | >100 RPM variance unable to correct |
Code P1515 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, P1515 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.