U0421 Allard K Diagnostic Trouble Code

U0421 Allard K code definition:
U0421 is a kind of OBD-II Diagnostic U-Network Diagnostic Trouble Code for Allard K.

U0421 Allard K is described as Fuel Rail Pressure Sensor Circuit Malfunction. This code is strictly about the incoming signal from the FRP (fuel rail pressure) sensor falling below the calibrated limit for a certain period of time. Other symptoms that may indicate this problem are when the Check Engine Light illuminates and when there is a visible lack of engine power.

Symptoms

Symptoms of a U0421 Allard K diagnostic trouble code may include:
  • Faulty O2 Sensor
  • Lack of power
  • If the DPF is not cleared the ECU will revert to the "Limp Home Mode" until the situation is corrected
  • Reduction in engine performance, especially at low RPM levels
Place of U0421 in engine

Possible Causes :

  • A faulty oxygen sensor. This will set a code in itself, however, a faulty oxygen sensor does not automatically condemn the sensor. The code just means that the sensor signal was not within specifications. An air leak or any of the above will cause an erroneous signal. There is a multitude of O2 codes relating to O2 performance which gives a clue to the problematic area.
  • Some aftermarket accessories and performance modifications
  • Fuel with a large percentage of sulfur will clog the DPF quickly

Common Misdiagnoses

  1. Crankshaft Position Sensor Circuit Performance Problem
  2. TP Sensor Learn Not Complete Problem
  3. Low Coolant Circuit Problem
  4. Cruise Control System - Vehicle Accel Too High Problem
  5. Kick-Down Switch Failed Short Problem

Repair Processes :

With the valve disconnected, the red voltmeter lead should be attached to the IMRC valve power wire and the black voltmeter lead connected to ground. You might continue by connecting the scanner to the vehicle diagnostic connector and retrieving all stored codes and freeze frame data. If the engine displays a miss and there wasn't a code, determine which cylinder is misfiring. If the exhaust manifold is visible, spay or pour a small amount of water on each cylinder exhaust port. Water will evaporate immediately on good cylinders and slowly on the missing cylinder. If this can't be accomplished pull the plugs and check the condition.

Process Tips

Diagnostic trouble code U0421 Allard K is not difficult to diagnose and repair. Thus, it always pays to have a qualified technician to look after your vehicle should this error code arise.

Diagnosing this code requires the engine to be in good running shape, without unmetered air entering the engine. If other codes that relates to misfiring condition, lean running or air entering the engine is present, then you must first fix those error codes before U0421 Allard K.

Fortunately, U0421 Allard K is a fairly common OBD code, meaning that it applies to all makes and models of vehicles made after 1996.

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