P0251 code definition
Injection Pump Fuel Metering Control "A" Malfunction (Cam/Rotor/Injector)
What the P0251 code means
P0251 is a generic fault code which will relate to one or more of the following components:
- optical sensor*
- crankshaft position sensor
- accelerator pedal position sensor
- intake air temperature sensor
These each provide input to the Engine Control Module (ECM) which results into an output from the ECM that adjusts fuel supply and ignition timing. The P0251 code indicates that there is an erratic reading portrayed from these components, so it is important to test each one. Your vehicle may not have all of the components listed, but the ones which it does should be tested.
*Generally, only diesel vehicles have an optical sensor, so if you don’t have a diesel, you may go ahead and almost completely rule that possibility out; rarely will a standard automobile have an optical sensor. The purpose of the optical sensor is to count the "pulses" emitted by the rotor (disc) in the injection pump. The rotor is silver colored, notched with two sets of impressions. For example(: :). The outlying notches provide the ECM with fuel pump speed information . The inner rotor uses 8 notches (one for each cylinder) to send the ECM cam reference info. If your vehicle is a diesel, the P0251 code is likely an indication that the optical sensor is sending erroneous readings. If your vehicle is not a diesel, you likely have a fault with one of the other components listed above.
What causes the P0251 code?
Contaminated, incorrect or bad gasoline
A dirty optical sensor
A clogged fuel pump, fuel filter or fuel injector
A faulty intake air temperature sensor, crankshaft position sensor, or accelerator pedal position sensor
A faulty fuel control actuator
A faulty Engine Control Module
Leaking fuel injector
A short to power or ground in the wiring related to the intake air temperature sensor, crankshaft position sensor, or accelerator pedal position sensor
Corrosion buildup on the connectors of the intake air temperature sensor, crankshaft position sensor,accelerator pedal position sensor, fuel injector or related wiring
What are the symptoms of the P0251 code?
- A slow or no start condition
- Smoke coming from the tail pipe
- Engine stall
- Misfire at idle
How does a mechanic diagnose the P0251 code?
Views the freeze frame data of the trouble code to determine the values of the optical sensor, crankshaft position sensor, accelerator pedal position sensor, and the intake air temperature sensor
Uses a scan tool to view the live data feedback of the optical sensor, crankshaft position sensor, accelerator pedal position sensor, and the intake air temperature sensor.
Uses a multimeter to check the voltage readings and resistance levels* of the optical sensor, crankshaft position sensor, accelerator pedal position sensor, and the intake air temperature sensor
Inspects the quality of the fuel
Performs a fuel pressure test
*The voltage and resistance of each component should meet the specifications of the manufacturer. The specifications will differ based on the year make and model of the vehicle. The specifications for your particular vehicle may be found through a site such as ProDemand or ask a mechanic
Common mistakes when diagnosing the P0251 code
There are many things which can cause the trouble code P0251. It is important to thoroughly test the components listed as the potential cause of the problem before just pointing one out as faulted. First find out which components are applicable for your vehicle. Then test the optical sensor, crankshaft position sensor, accelerator pedal position sensor, and the intake air temperature sensor, as found applicable.
How serious is the P0251 code?
Other than a potential no start or stall condition, it is generally not extremely serious. However, keep in mind that with a Check Engine Light being on, you can not legally pass inspection.
What repairs can fix the P0251 code?
Replacing a faulty accelerator position sensor
Replacing a faulty intake air temperature sensor
Replacing a faulty optical sensor
Cleaning a dirty optical sensor
Using a fuel treatment which will help clean out deposits or debris within the fuel system
Replacing faulty glow plugs (diesel only)
Repairing damaged or frayed intake air temperature sensor wiring
Repairing an open, short to power or ground in the intake air temperature sensor circuit
Repairing of short, open to power or ground in the accelerator position sensor circuit
Repairing open, short to power or ground in the crankshaft position sensor circuit
Repairing a short, open to power or ground with the wiring related to the optical sensor
Additional comments for consideration regarding the P0251 code
Keep in mind that after changing a faulty optical sensor, a scan tool must be used to re-learn the position of the cam references.
Need help with a P0251 code?
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