U0121 Checker Aerobus Diagnostic Trouble Code

U0121 Checker Aerobus code definition:
U0121 is a kind of OBD-II Diagnostic U-Network Diagnostic Trouble Code for Checker Aerobus.

U0121 Checker Aerobus is defined as Cylinder 3 Injector Circuit Malfunction. This code means the PCM (powertrain control module, also known as ECM or engine control) has detected a problem in the injector or its wiring, or in injector circuit for cylinder 3. If the car doesn't start, they will proceed by testing the voltage at the fuel pump connector and see if the fuel pump relay, fuse, and PCM circuitry should be inspected.

Symptoms

Symptoms of a U0121 Checker Aerobus diagnostic trouble code may include:
  • Faulty or damaged wiring
  • 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
  • Poor fuel economy
  • Engine surge
Place of U0121 in engine

Possible Causes :

  • Rarely - faulty Powertrain Control Module (PCM-programming required after replacement)
  • Rough running engine due to misfiring plugs. Many engines have misfire codes to indicate the cylinder effected, such as P0307 for number 7.
  • A large vacuum leak would cause a massive amount of un-metered air to enter the intake manifold resulting in an overly lean mixture.

Common Misdiagnoses

  1. TP Sensor Learn Not Complete Problem
  2. Low Coolant Circuit Problem
  3. Cruise Control System - Vehicle Accel Too High Problem
  4. TFP Valve Position Switch-Drive Without Drive Ratio Problem
  5. Problem

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. It should be pretty close to battery voltage (12 volts). If not, then this is the circuit that has the problem. 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 U0121 Checker Aerobus 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 U0121 Checker Aerobus.

Fortunately, U0121 Checker Aerobus is a fairly common OBD code, meaning that it applies to all makes and models of vehicles made after 1996.

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