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Fault Code 21 - O2 Sensor Stays at center or shorted to voltage

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Code 21 indicates a problem in the oxygen sensor circuit. The oxygen sensor is an electrochemical device consisting of two layers of platinum separated by a layer of zirconium oxide. One plate is exposed to ambient oxygen while the other is exposed to the oxygen content of the exhaust system. When the engine is started, the hot exhaust gases pass by the oxygen sensor. When the sensor temperature reaches 600° F, it becomes conductive for oxygen ions. If the amount of oxygen electrons on the exhaust side are equal to the amount on the ambient air side, then the electrons equalize and no voltage is produced. As the oxygen content of the exhaust decreases, an imbalance occurs and the oxygen sensor begins to produce a voltage.

Normal operating voltages for the oxygen sensor range from a low of 100 millivolts to a high of 900 millivolts. The voltage produced can occasionally be higher or lower than these figures. About halfway between these voltages is 450 mv - known as the crossover point. Any voltage less than 450 mv is interpreted by the computer as excessive oxygen caused a lean exhaust condition; any voltage greater than 450 mv is interpreted by the computer as insufficient oxygen caused by a rich exhaust condition. Anything that affects the content of oxygen in the exhaust will affect the oxygen sensor signal.

When the oxygen sensor voltage is indicating a lean condition, the computer will respond by enriching the mixture. When the oxygen sensor voltage is high, the computer will respond by leaning out the mixture. In this manner, the computer adjusts for minor errors and variations from the rest of the input sensors and controls the air fuel ratio at 14.7: 1.

The oxygen sensor itself is a very dependable unit. Occasionally the oxygen sensor will fail with an internal open circuit. The most common problem that occurs with oxygen sensors is contamination. There are three common sources of contamination for the oxygen sensor:

  1. RTV silicone - a commonly used sealer and adhesive. The fumes from this adhesive can coat the oxygen sensor, slowing its ability to respond to changes in the oxygen content of the exhaust. When the oxygen sensor becomes con- taminated with silicone, it delivers a lean signal to the EEC computer.
  2. Leaded fuel - not readily available now. When the oxygen sensor becomes contaminated by leaded fuel, the output voltage will be stuck above the crossover point. This is a rich exhaust indication. Although this rarely occurs by accident, be sure to check the contents of any fuel additives being used to ensure that it does not contain tetra-ethyl lead.
  3. Soot contamination - The two primary sources are oil and fuel. Oil soot is a result of a mechanical engine condition such as worn rings or valve guides. Fuel soot is either the result of a rich-running engine or a misfire. Either form of soot impedes the operation of the oxygen sensor. The output voltage remains constant at around 0.5 volts.

Testing the sensor:

Most Rams:
  • left sensor - black/darkgreen wire
  • right sensor - tan/white wire
  • ground - black/light blue
1996 models with pre cat and post-cat sensors:
  • Pre-cat sensor - tan/white wire
  • Post-cat sensor - orange/black

First, try this simple test of the oxygen sensor. With the oxygen sensor connected, start the engine and allow it to run at 2,000 rpm for two minutes. Connect a digital voltmeter between the signal wire and ground. With the engine idling, watch the voltage reading on the voltmeter. You should observe the voltage toggling above and below 0.450 volts. If the voltage is stuck high, or in the middle, create a vacuum leak and look for a response. If the voltage is stuck low, or in the middle, goose the throttle and look for a response. If there is no response in the voltage, replace the oxygen sensor.

A more thorough test of the oxygen sensor circuit involves two steps.

  1. Disconnect the connector between the oxygen sensor and the computer. Connect a high-impedance voltmeter between the oxygen sensor and ground. Start the engine and allow it to run until the upper radiator hose is hot and pressurized. Hold the oxygen lead to the computer with one hand and touch the positive battery terminal with the other hand. This delivers a small voltage to the computer, causing the computer to believe that the engine is running rich. The engine rpm and the oxygen sensor voltage should drop.
  2. If there is no response, connect the oxygen wire that goes to the computer to ground. This makes the computer believe that the engine is running lean. The rpm and the oxygen sensor voltage should both rise.

If the system passes either test, then the system is working properly. If the rpm changes but the oxygen voltage does not, replace the oxygen sensor. If neither changes, check for continuity on the wire to the PCM. If the wire is good and the connection is good, replace the PCM.


This page would not have been possible without the help of Hank LaViers,
who graciously loaned me his extensive Mopar engine library 
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Last Update: August 8, 2001