In the early 1980's, computer controls were installed in vehicles to meet Federal Government regulations for lower emissions and better fuel economy. These engine computers use a variety of sensors to determine the operating conditions of the engine and the appropriate fuel rate needed to produce engine power while meeting emissions standards. Oxygen (O2) sensors are mounted in the the exhaust stream(s) to measure the oxygen remaining in the exhaust after combustion. This information is used by the engine computer to adjust the air/fuel ratio entering the engine.
O2 Sensor variations
*Sensors with 3 or 4 wires have an internal heater.
*Sensors with one wire or three wires use the sensor body as ground
*Sensors with two wires or four wires are grounded by the wiring harness.
*Sensors with five wire are WIDE band type, OSP/ EFIE can not manage them
Two types of oxygen sensor elements are commonly used.
How does an oxygen sensor work?
The electrode of an O2 sensor converts the measured oxygen level in the exhaust gas to a voltage, which the engine computer inputs and compares to ideal values. Voltage output from the sensor varies from 0.1V (High Oxygen, Lean mixture) to 0.9 V (low oxygen, rich mixture). The computer constantly reads the O2 sensor and adjusts the fuel injection rate by varing the pulse width of the fuel injector "open" time. An air/fuel mixture of 14.7:1 is considered ideal for emissions and allows the catalytic converter to operate at peak efficiency.
An oxygen sensor produces no voltage when the sensor temperature is below 600 degrees F. Until the sensor reaches operating temperature, the engine computer operates in open loop mode where default fuel settings are used. The sensor used in the Dodge Ram is internally heated to speed warm-up.
What happens when the sensor fails?
When the engine reaches normal operating temperature and/or has been running for more than two minutes, the computer will vary the mixture and monitor the O2 sensor. If the sensor produces a steady signal below 0.45V with the engine running above 1500 rpm, the sensor will be flagged as bad and a fault code 51 or 52 will be set. Any other problem detected in the O2 sensor circuit will set a fault code 21.
When the O2 sensor fails, the computer runs in open loop mode and the fuel mixture to the engine will not be correct. This can cause the engine to ping, run rich or lean, and/or miss.
Unleaded fuel is essential to the proper and continued operation of the O2 sensor. Never use cleaning solvents of any kind on the O2 sensor, solvents can permanently contaminate the sensor element.
