What does it mean?
The module has calculated that less fuel is actually reaching the combustion chambers than it commanded from the A fuel injection system. It arrives at this conclusion by watching how much correction the fuel system needs to hold the target rail pressure and how the delivered quantity compares with the modelled requirement for the current load. When the correction reaches its limit in the direction of adding fuel and the shortfall persists, the code is stored. In practice this almost always means a restriction or a leak somewhere between the tank and the injector tips, or a high-pressure pump that can no longer deliver its rated volume. Because the engine is being starved, the module normally limits power to keep combustion stable.
Symptoms
- Hard starting and extended cranking, worse when the tank is low
- Clear loss of power, particularly under load, on hills or when accelerating hard
- Hesitation, stumbling or a sudden drop into limp mode when demand rises
- The engine may stall at high load or cut out and restart
- MIL on, frequently together with rail pressure codes
- Occasional white smoke from incomplete combustion
Likely causes
- Clogged fuel filter, which is the most common single cause and often overdue for replacement
- Weak lift or transfer pump, or a blocked strainer inside the tank
- Air entering the suction side through a loose union, a cracked line or a perished seal
- Sticking fuel metering or volume control valve on the high-pressure pump
- Worn high-pressure pump that can no longer generate the required flow
- Excessive injector return leakage, so much of the delivered fuel goes back to the tank
- Kinked, crushed or restricted fuel line, or a blocked tank breather creating a vacuum
- Waxed or gelled diesel in very cold weather, or contaminated fuel
How to fix it
- Check the fuel filter service history and inspect the filter and water separator before anything else
- Measure supply pressure and vacuum on the suction side of the high-pressure pump while cranking and under load
- Fit a clear section of hose or use a sight glass to look for air bubbles being drawn in
- Log actual against desired rail pressure at idle, on a snap acceleration and under sustained load
- Perform an injector leak-off test to find excessive return flow
- Command and monitor the fuel metering valve duty cycle to see whether it responds normally
- Inspect the tank and lines for contamination, restriction and crushed sections, and check the tank breather
- Renew the worn component the tests identify, then confirm rail pressure follows the target through the whole load range