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Why use an oxygen sensor?

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.
• 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.

Testing the O2 Sensor

With the key on, check for battery voltage to the O2 sensor connectors.

• The dark green/orange wire should have 12 volts.
• The Black/white (may be black/tan) wire is the ground.

Insert a pin into back of the O2 sensor connector to contact the signal wire:

• Connect the positive probe of a voltmeter to the signal lead.
• Connect the negative probe of the voltmeter to ground.
• Start the engine and monitor the voltage (millivolts) as the engine warms up.
• When the engine is cold, the O2 sensor should output 0.1V to 0.2V.
• After about 20 minutes, the engine should reach operating temperature and the sensor output should fluctuate between 0.1V and 0.9 volts, and the computerwill run in closed loop mode.
• If the engine never enters closed loop or takes a long time to enter closed loop mode, replace the sensor.

Other manuals say you can use an un-lit propane torch to briefly put propane in the V8's air cleaner inlet to test to see if the still installed O2 sensor will go rich by going up to .9 volts. Wear safety glasses and stand clear of any potential 'backfire' through the throttle body that might happen !

Ram Upstream Oxygen Sensor Goal Voltage
From Star Center News #5, February 2000 - tests for those who have access to OBD II or OBD III code readers.

    Since the introduction of OBDII with both upstream and downstream O2 sensors, the downstream O2 sensor has been used to evaluate catalytic converter operation and to control converter efficiency.
    When looking at the upstream goal voltage, it should fluctuate above and below the normal center (.4 to .6 volts) based upon what the downstream O2 sensor is reading. The vehicle should be driven through all speed ranges when checking goal voltage. If the goal stays high or low all the time, a thorough check of the downstream oxygen sensor should be made. Be sure you check the sensor, wiring harness, connectors, catalytic converter, and inspect for exhaust leaks.
   Although a MIL lamp may not be illuminated, there are instances when the downstream O2 sensor goal voltage is out of range, causing driveability problems or low fuel economy.
   If the downstream O2 sensor tends to stay below center all the time, the PCM will adjust the goal voltage higher in an efforet to correct the low O2 readings of the downstream sensor. This can result in low fuel mileage.
   If the downstream O2 sensor has a tendancy to stay above center, the PCM will set a low goal voltage which results in a lean fuel mix which can cause the engine to run hotter and aggravate a spark knock condition.
   Sometimes a service technician will reset the PCM and take it for a test drive. The PCM does a recalibration after the reset, and the probelm may go away until the PCM has completed a relearn of the system and then begins to attempt to correct for the downstream sensor error. If the symptoms reappear after several days of driving after a service is performed, check the downstream O2 sensor, the exhaust system, the wiring, and the catalytic converter. Note: a worn out catalytic converter can produce symptoms similar to a bad downstream O2 sensor.

Last Update: July 9, 1999