UEGO sensors operate differently than traditional oxygen sensors. UEGO sensors are wide-band oxygen sensors containing a pump cell operated by the Powertrain control module (PCM). The PCM uses the current required to operate the pump to determine the air fuel ratio. Common fault codes you will see with these sensors are P0130 or P0150. Be sure to check for PCM software updates before replacing or repairing a problem with a sensor.
UEGO sensors are designed with a reference air cell, a Nernst cell and a measurement cell. The sensor compares exhaust gas oxygen content with the content of the reference cell.
The oxygen for the reference chamber is supplied by pumping oxygen ions from the measurement chamber into the reference chamber. The Nernst compares the oxygen content of the exhaust to that of the reference chamber. As exhaust gas becomes rich or lean oxygen is pumped in and out of the measurement chamber to maintain a mixture of 1.0 lambda. The amount of oxygen needed to do this varies depending on the air fuel ratio. The PCM measures the current of the pump cell required to maintain the correct mixture and estimates the air fuel ratio from this. The measured air fuel ratio used by the PCM is not the output from the sensor, but the current used by the PCM to control the sensor pump. When the exhaust is lean, the PCM pumps oxygen out of the pump cell by applying a positive current to the pump cell. When the exhaust is rich, the PCM pumps oxygen into the pump cell by applying a negative current to the pump cell.
You can monitor sensor voltage and operation at the UEGO sensor electrical connector, as you would with a narrow-band oxygen sensor. You will not see it switch as rapidly as you would a narrow-band oxygen sensor (4-5 times a second).You can still however check the voltage to confirm the current air fuel ratio as well as force the sensor rich and lean to test it.
The sensor will have six wires, two for the heater circuit and four for the oxygen sensor circuit.
When testing with a scope, the voltage displayed will depend on where you place the tester negative test lead. If on battery negative, you will see voltages above 2.0 volts at the sensor or pump circuit. If using the sensor floating-ground, you will +/- 0.450 volts at the sensor or pump circuit.
The signals displayed are the sensor and pump signals, the pump signals mirror each other quite closely while the sensor holds steady. The scope channels are connected in the following order: channel 1 yellow trace on the pump cell. Channel 2 red trace on measured pump cell and Channel 3 green trace on the universal sensor. Pump and measured pump circuit voltage is fluctuating from 2.1 – 2.7 volts. The universal sensor holds steady at 2.9 volts.
When we move our scope negative test lead to the sensor ground, the sensor and pump signals go to +/- 350 millivolts. Once at 350 millivolts, sensor fluctuation is more visible and easer to test.
Begin by forcing the sensor lean (create a large vacuum leak), voltage should go to and remain at +400 millivolts when the vacuum leak is present.
Then force the sensor rich (enrich engine with propane enrichment tool), voltage will go to -400 millivolts.
If the sensor does not respond to mixture forced-adjustments it is likely faulty and should be replaced. Be sure to confirm there are no vacuum, fuel or exhaust leaks that can affect sensor operation before replacing.
