Technical Description of OBD-II Trouble Code
This trouble code refers to an intermittent issue with the Injection Pump Fuel Metering Control B, specifically related to the Cam/Rotor/Injector. It is commonly found in diesel fueled OBD-II equipped engines, including certain Mercedes Benz and VW vehicles. The repair steps may vary depending on the specific year, make, model, and powertrain configuration of the vehicle.
The Injection Pump Metering Control Circuit “B” is typically located inside or on the side of the injection pump, which is bolted to the engine. This circuit consists of a Fuel Rack Position (FRP) Sensor and a Fuel Quantity Actuator. The FRP sensor converts the amount of diesel fuel being delivered by the Fuel Quantity Actuator into an electrical signal for the Powertrain Control Module (PCM). The PCM uses this signal to determine the appropriate amount of fuel to inject into the engine based on its operating conditions. If the input from the FRP sensor does not match the normal engine operating conditions stored in the PCM’s memory, even for a brief moment, this trouble code is triggered. The PCM also checks the voltage signal from the FRP sensor during initial Key On to ensure its accuracy.
The code P0255 Injection Pump Fuel Metering Control B Intermittent (Cam/Rotor/Injector) can be caused by mechanical issues, such as problems with the EVAP system, or electrical issues related to the FRP sensor circuit. It is important not to overlook these potential causes, especially when dealing with an intermittent problem. Consult a vehicle specific repair manual to determine the specific “B” part of the circuit for your particular application. Troubleshooting steps may vary depending on the manufacturer, type of FRP sensor, and wire colors.
Severity of this DTC
The severity of this trouble code depends on the nature of the failure. If it is a mechanical failure, it is considered severe. However, if it is an electrical failure, it is not as severe because the PCM can compensate adequately for it.
Symptoms of the code
Some symptoms of a P0255 trouble code may include:
Common causes of the code
Some common causes for this P0255 code may include:
Troubleshooting steps for P0255
A good starting point is to check for technical service bulletins (TSB) specific to your vehicle. There may be a known issue with a known fix provided by the manufacturer, which can save time and money during diagnosis.
Next, locate the FRP sensor on your vehicle, which is usually mounted inside or on the side of the injection pump bolted to the engine. Inspect the connector and wiring visually, looking for any signs of scraping, rubbing, bare wires, burn spots, or melted plastic. Disconnect the connector and carefully inspect the terminals inside for any signs of burning or corrosion. Clean the terminals if necessary using electrical contact cleaner and a plastic bristle brush, then apply electrical grease where the terminals make contact.
If you have a scan tool, clear the diagnostic trouble codes from memory and see if the P0255 code returns. If it does not, the issue was most likely related to the connections.
If the P0255 code does return, further testing of the FRP sensor and its associated circuits is necessary. With the Key Off, disconnect the electrical connector at the FRP sensor and use a Digital Voltmeter to check the voltage at the power and ground terminals according to the manufacturer’s specifications. If the voltage does not meet the specifications, repair the power or ground wire, or replace the PCM.
If the power and ground tests pass, the signal wire needs to be tested. With the connector still disconnected, use the Digital Voltmeter to check the voltage at the signal wire terminal. It should read 5 volts. If not, repair the signal wire, or replace the PCM.
If all previous tests have passed and the P0255 code persists, it is likely that the FRP sensor or Fuel Quantity Actuator has failed. However, a failed PCM cannot be ruled out until the FRP sensor or Fuel Quantity Actuator has been replaced. If unsure, seek assistance from a trained automotive diagnostician. Note that PCMs must be programmed or calibrated to the vehicle for correct installation.
