Jeep Wagoneer/Commando/Cherokee 1984-1998

Heated Oxygen (HO2S) Sensor



The heated oxygen sensor, or HO2S sensor is located at the exhaust system, usually near the catalytic converter. It produces a voltage signal of 0.1-1.0 volts based on the amount of oxygen in the exhaust gas. When a low amount of oxygen is present (caused by a rich air/fuel mixture), the sensor produces a low voltage. When a high amount of oxygen is present (caused by a lean air/fuel mixture), the sensor produces a high voltage. Because an accurate voltage signal is only produced if the sensor temperature is above approximately 600°F, a fast acting heating element is built into its body.

The ECU uses the HO2S sensor voltage signal to constantly adjust the amount of fuel injected which keeps the engine at its peek efficiency.


  1. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
  3. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the HO2S sensor signal wire and battery ground.
  5. Verify that the sensor voltage fluctuates rapidly between 0.40-0.60 volts.
  7. If the sensor voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the voltage fluctuates very slowly between the specified range (H02S signal crosses 0.5 volts less than 5 times in ten seconds), the sensor may be faulty.
  9. If the sensor voltage stabilizes at either end of the specified range, the ECU is probably not able to compensate for a mechanical problem such as a vacuum leak. These types of mechanical problems will cause the sensor to report a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the H02S sensor test repeated.
  11. Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the sensor to detect a lean mixture condition. Reattach the vacuum hose.
  13. Richen the mixture using a propane enrichment tool. Sensor voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the sensor to detect a rich mixture condition.
  15. If the sensor voltage is above or below the specified range, the sensor and/or the sensor wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.
  17. Further sensor operational testing requires the use of a special tester M.S.1700, or equivalent.