Fuel Rail Pressure Sensor Circuit Malfunction
The engine's computer can't properly read the fuel pressure sensor, like a broken fuel gauge that gives incorrect readings. This prevents the engine from adjusting fuel delivery correctly, causing performance issues.
The ECU monitors voltage signals from the fuel rail pressure sensor to maintain proper fuel injection timing and quantity. The sensor should produce a voltage signal proportional to fuel pressure, typically 0.5-4.5 volts corresponding to 0-100 PSI. If voltage is out of range or unstable, the ECU sets a fault code.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage | 0.5-4.5 volts (0-100 PSI fuel pressure) | Below 0.5V or above 4.5V, or no signal detected |
| Pressure Signal Stability | Steady signal with normal fluctuations | Erratic, intermittent, or constant deviation |
Code P0191 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, P0191 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.