P0421 Diagnostic Trouble Code

P0421 is a kind of OBD-II Diagnostic P-Powertrain Diagnostic Trouble Code.

It is considered generic because it applies to all makes and models of vehicles, although specific repair steps may be different depending on your car model.

Symptoms

Symptoms of a P0421 diagnostic trouble code may include:

Rich or lean exhaust
Fuel pressure too low
Engine surge
Hesitation upon acceleration

Possible Causes :

Some aftermarket accessories and performance modifications
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.
Faulty IMRC actuator relay (if equipped)

Repair Processes :

Replace the oxygen sensor that is downstream from the catalytic converter on the side of the engine that does not contain cylinder #1. Also, if a oxygen sensor code stating "heater circuit malfunction" the sensor has most probably failed. Look for missing ground strap or a rusty ground where the fuel tank is grounded to the frame. Check for damage to the wiring harness connector. Pack the connector cavity with dielectric silicone compound (same stuff they use for light bulb sockets and spark plug wires) and reassemble.

Process Tips

Diagnostic trouble code P0421 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 P0421.

Fortunately, P0421 is a fairly common OBD code, meaning that it applies to all makes and models of vehicles made after 1996.

P0421 Trouble Code For Specific Cars

P0421 Nissan Kubistar Code Word Unregestered
P0421 Seat Toledo Input/Turbine Speed Sensor Circuit No Signal
P0421 Land Rover Range Rover Steering VAPS II Circuit Loop Short To Ground
P0421 Maserati Levante Transmission Fluid Pressure Sensor/Switch F Circuit
P0421 Fiat Croma Lamp Keypad Output Open Circuit
P0421 Kia Xedos Servo Motor Foot Circuit Failure
P0421 Holden Eh Persistent Misfire