Tip#25: Check engine light code 51 for 94 Dodge Spirit that causes rotten smell

Obd1 code 51 is an oxygen sensor circuit failure that makes the voltage reading stuck at a lower range (between 0.1 to 0.4 volt) showing lean conditions. When this happens, there is incomplete combustion causing Sulphur from the gasoline to mix with Oxygen molecules resulting in rotten egg smell at your exhaust. Diagnosis to this problem is detailed below.

Symptom:

Code 51 showing lower voltage readings instead of the fluctuating normal voltage.

Basic Concept:

This car has a 3 L engine and an oxygen sensor with 4 wires at the exhaust. Two wires are for the heater circuit and the other two is for the fuel mixture. As discussed in other previous blogs, the voltage reading can be manipulated by manually adding fuel or more air to the intake system. During this time, the voltage at the sensor signal wire should change and oscillate. Otherwise, the sensor itself has failed.

Problem will be get complex if after replacing the sensor; it fails again after a few weeks which suggest only one thing: CONTAMINATION. Causes? There are some engine self induced factors that will cause premature failure of the sensor including:

Coolant contamination caused by head gasket or cracked cylinder head.
Oil contamination caused by oil dripping directly to the sensor from the steering switch.

Both problems can be tested and checked visually using proper tools. Use Dodge service manual or ATS.