Fuel Pulse In Range But Lower Than Expected
The fuel injector is firing, but not delivering as much fuel as the engine computer expects. Think of it like a water faucet that's turned on but only trickling instead of flowing normally.
The ECU monitors fuel injector pulse width (how long the injector stays open) and expects a certain amount of fuel delivered based on engine load, RPM, and airflow. When the actual fuel delivery falls short of the calculated requirement, this fault triggers. The computer detects the injector is pulsing but the oxygen sensors indicate a lean condition.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Fuel Injector Pulse Width | Expected duration based on load/RPM calculation | Measured pulse significantly lower than commanded value |
| Air-Fuel Ratio | 14.7:1 stoichiometric | Leaner than target, indicating insufficient fuel delivery |
Code P1291 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, P1291 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.