Oxygen Sensor Diagnosis. [Question: How do I diagnose oxygen sensor problems?] This is from Bosch information:

Test for a rich mixture as follows:

Disconnect the sensor lead to the control unit.

Run the engine at 2500 rpm

Artificially enrich the fuel mixture on electronic fuel injected engines by removing and plugging the vacuum line to the fuel pressure regulator.

If the voltmeter rapidly reads .9 volts, then the oxygen sensor is correctly sensing a rich mixture. But, if the voltmeter responds sluggishly, or if it stays below .9 volts, try running it at 3,000 rpm for a few minutes, then check again. No improvement means you buy a new sensor.

Test for a lean mixture as follows:

Induce a small vacuum leak

If the voltmeter rapidly drops to .2 volts or below in less than a second, then the oxygen sensor is correctly measuring the lean mixture. But if the voltmeter responds sluggishly, of if it stays above .2 volts, give it the 3,000 rpm treatment and try again. If no improvement, then the sensor should be replaced.

Test dynamic performance as follows:

Reconnect the sensor lead and tap your meter into the signal wire

Set the mixture to specification.

Run the engine at 1500 rpm.

You should see rapidly changing readings that average somewhere around .5 volt as the computer keeps adjusting the blend. The sensor output should fluctuate around .5 volts. If it doesn't, replace the sensor. Deciding whether or not response is slow enough to justify replacement requires some judgment. A common rule of thumb for minimum activity is eight trips across the rich/lean line in ten seconds, and sometimes you can find specs for cross-counts.