P1A78 Mercury Mc4 Diagnostic Trouble Code

P1A78 Mercury Mc4 code definition:
P1A78 is a kind of OBD-II Diagnostic P-Powertrain Diagnostic Trouble Code for Mercury Mc4.

P1A78 Mercury Mc4 is described as Fuel System Too Rich, which basically means bank 1's oxygen sensor has detected a rich condition of fuel. This means there's not enough oxygen in the exhaust from too much fuel being delivered into the air and gas mixture for combusion. So if the problem persists despite of your diagnosis, then you should never hesitate to take your vehicle to your dealer or to a qualified mechanic.

Symptoms

Symptoms of a P1A78 Mercury Mc4 diagnostic trouble code may include:
  • Fuel pressure too high
  • Diminished fuel efficiency
  • Rich or lean exhaust
  • The light may stay on or light intermittently as the DPF regenerates. The engine will be sluggish on acceleration.
Place of P1A78 in engine

Possible Causes :

  • Low fuel pressure caused by a clogged filter, failing fuel pump, failed fuel pressure regulator or clogged or leaking injectors.
  • Low fuel pressure caused by a clogged filter, failing fuel pump, failed fuel pressure regulator or clogged or leaking injectors.
  • Faulty IMRC actuator relay (if equipped)

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 :

Find out what kind of system you have and verify that voltage to the fuel level sensor is present at the fuel pump wiring harness. 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.

Process Tips

Diagnostic trouble code P1A78 Mercury Mc4 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 P1A78 Mercury Mc4.

Fortunately, P1A78 Mercury Mc4 is a fairly common OBD code, meaning that it applies to all makes and models of vehicles made after 1996.

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