U0123 Diagnostic Trouble Code

U0123 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 U0123 diagnostic trouble code may include:

Poor acceleration
Cherry red hot catalytic converter
Possible spark knock (detonation / pre-ignition)
If the DPF is not cleared the ECU will revert to the "Limp Home Mode" until the situation is corrected

Possible Causes :

A large air leak at or near the number one oxygen sensor would also cause a lean mixture.
Faulty IMRC actuator relay (if equipped)
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.

Repair Processes :

If the scan tool has that capability, and the scan tool can operate the IMRC valves, then the problem is either resolved and a simple code clearing is all that's left or a new PCM would be called for. 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.

Process Tips

Diagnostic trouble code U0123 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 U0123.

Fortunately, U0123 is a fairly common OBD code, meaning that it applies to all makes and models of vehicles made after 1996.

U0123 Trouble Code For Specific Cars

U0123 Morris Ital Brake Pressure Switch Mechanical Failure
U0123 Allard 2.25-litre Air Bag Crash Sensor #2 Ground Circuit Short To Battery
U0123 Cadillac Osceola Dim Panel Decrease Input Circuit Short To Ground
U0123 Fso Polonez O2 Sensor Circuit High Voltage (Bank 1 Sensor 3)
U0123 Locomobile 40 Timing Reference High Resolution Signal A Malfunction
U0123 Mercedes Cl 600 ABS Inlet Valve Coil RF Circuit Short To Battery
U0123 Tornado Talisman Motor # 2 Input Circuit Failure