U1977 Mercedes C 300 Diagnostic Trouble Code

U1977 Mercedes C 300 code definition:
U1977 is a kind of OBD-II Diagnostic U-Network Diagnostic Trouble Code for Mercedes C 300.

U1977 Mercedes C 300 shows up when there's an electrical problem that causes the fuel volume regulator to malfunction and open. It's quite an unusual problem and likely intermittent. There are two wires dedicated for each coil; one is battery feed which usually comes from the distribution center, and the other one for the coil driver circuit from the PCM. The PCM grounds or ungrounds the circuit to activate or deactivate the coil.

Symptoms

Symptoms of a U1977 Mercedes C 300 diagnostic trouble code may include:

If the DPF is not cleared the ECU will revert to the "Limp Home Mode" until the situation is corrected
Corroded or damaged connector
Reduction in engine performance, especially at low RPM levels
Malfunction Indicator Lamp (MIL) illuminated

Possible Causes :

Fuel with a large percentage of sulfur will clog the DPF quickly
The Mass Airflow sensor will also cause this problem. Mass Airflow circuit malfunction.
Dirty air cleaner element Damaged DPF

Common Misdiagnoses

Control Module Throttle Actuator Position Performance Problem
Air Conditioning (A/C) Clutch Feedback Circuit High Voltage Problem
Cruise Vehicle Speed/Set Speed Difference Too High Problem
Kick-Down Switch Failed Short Problem
Park/Neutral Postion (PNP) Switch Circuit Engine Torque Signal Circuit 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. 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. Use the DVOM to test resistance and continuity on all system circuits if actuator and sensor resistance levels are in compliance with manufacturer's specifications.

Process Tips

Diagnostic trouble code U1977 Mercedes C 300 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 U1977 Mercedes C 300.

Fortunately, U1977 Mercedes C 300 is a fairly common OBD code, meaning that it applies to all makes and models of vehicles made after 1996.