Fuel Rail Pressure Sensor Performance DTC P0191 - keywords 6.6 diesel engine LBZ LLY LMM SES

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

A dealer may encounter a customer vehicle with a Diagnostic Trouble Code (DTC) of P0191. The DTC may set shortly after the vehicle is started. P0191 may set due to actual fuel rail pressure not dropping quickly enough when the engine is shut off.

Complete the current SI diagnostic for any trouble code or symptom found. The information in this PI would only apply if DTC P0191 is setting due to slow fuel rail pressure drop.

Important: If the vehicle is equipped with a 2007-2008 6.6L RPO code LMM engine, there has been a new calibration released to repair the P0191. Install the latest TIS2000 calibration and evaluate the repairs. If the vehicle has a LLY or LBZ engine, continue with the information in this PI.

If the SI diagnostic is inconclusive, or DTC P0191 sets on every restart, monitor the fuel rail pressure when the engine is shut off. If the actual fuel rail pressure does not drop below 6.6 Mpa within 30 seconds, this code will set.

This slow bleed down is the result of harmless fuel deposits in the fuel injectors. This code usually sets when the vehicle is driven a short distance and shut off. Compare actual fuel pressure readings to a known good vehicle if possible.

If a slow to drop fuel rail pressure (as described in this PI) has been verified, replace the fuel filter. During the fuel filter replacement, fill the fuel filter with GM Diesel Fuel Additive (found in the newest version of bulletin 03-06-04-017D) Run the vehicle to operating temperature, and evaluate the fuel rail pressure drop/resetting of dtc P0191.

This PI will be updated when new information becomes available.

Note: Replacement of the fuel rail pressure sensor or ECM will not repair the vehicle if the condition is occurring as described in this PI.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.