U0021 Diagnostic Trouble Code

U0021 is a kind of OBD-II Diagnostic U-Network 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 U0021 diagnostic trouble code may include:

Fuel pressure too low
Rich or lean exhaust
Diminished fuel efficiency
The engine oil will exhibit dilution from the ECUs attempts to increase engine temperature. Some vehicles advance the fuel injection timing slightly after top center to afterburn a small amount of fuel to increase exhaust temperatures. Some of this fuel enters the crankcase. When the ECU determines the necessity to regenerate the DPF, the service life of the oil be shortened significantly

Possible Causes :

Defective ECU strategies or components will prevent proper regeneration
The Mass Airflow sensor is a hot wire that senses the volume of air entering the intake manifold. The computer uses this information to control fuel mixture.
Faulty IMRC actuator/sensor

Repair Processes :

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. Look for rubbing, scraping, bare wires, worn spots or melted plastic connectors. Pull the connectors apart and carefully look at the terminals (the metal parts) inside the connectors. 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.

Process Tips

Diagnostic trouble code U0021 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 U0021.

Fortunately, U0021 is a fairly common OBD code, meaning that it applies to all makes and models of vehicles made after 1996.

U0021 Trouble Code For Specific Cars

U0021 Geely Merrie Octane Adjust Pin Out Of Self Test Range
U0021 Ferrari 212 Injector High Side Short To GND Or VBATT - Bank 2
U0021 Bmw M6 Clutch Pedal Switch B Circuit
U0021 Proton Gen2 Fuel Sender Circuit Short To Battery
U0021 Sunbeam-talbot 4-litre Mass or Volume Air Flow Circuit low Input
U0021 Mazda Xedos 6 A/C Post Heater Sensor #2 Circuit Short to Ground
U0021 Lada 2105 Shift Solenoid A Electrical/1-2 Shift Solenoid Circuit Electrical