Questions and Answers

Plymouth Voyager Code 21 Q. Vince, I really appreciate you making yourself available to help us shade tree mechanics. I've gone through all the question archives, and didn't see an answer to my question. Purchased mini-van at 92,000 miles. Dealer replaced Oxygen Sensor at 93,000 miles. Transmission rebuilt at 95,000 miles, EGR valve replaced at 113,000 miles. "Turn key" idiot light code showed 21, Oxygen Sensor, so I replaced it at 142,300 miles.

It was okay for two days, only several local trips, then "Check Engine Light" came on again. Again Code showed 21. I disconnected the battery to reset the computer (right or wrong), and after a day, two short trips, the light came on again upon cold starting.

I find that if I start it cold, and let it idle for a few minutes, then turn the ignition off, when I restart immediately, the light doesn't come on again, until the next cold start. Is it possible that this engine uses two Oxygen Sensors? Or do I need to take the car to a REAL mechanic ?

• 1994 Plymouth Voyager SE
• 3.3 liter V-6
• Automatic Transmission
• 142,500 miles
• Fuel injection

Thanks, very much.
Dennis

A. Code 21 - "HO2S stays centered. Neither rich nor lean condition is detected from the oxygen sensor input." Or "HO2S shorted high. Oxygen sensor input voltage maintained above normal operating range".

You will need a high impedance, Minimum 10 Meg (10,000,000) ohm input impedance, Digital Volt Ohm Meter (DVOM) to test the HO2S.

1. Make sure the vehicle is at normal operating temperature. Run the engine at 2,500 rpm for 3 minutes with the transmission in PARK (A/T) or NEUTRAL (M/T).

CAUTION: To avoid damaging sensor, voltmeter must be set in Volts mode only, (not ohms).

2. With engine OFF, connect the negative lead of the DVOM to engine ground. Set the DVOM to read DC VOLTS. Disconnect the oxygen sensor (O2S) electrical connector and connect the positive lead of the DVOM to the oxygen sensor signal wire (black/dark green).
3. Start engine. Create a lean condition for 10 seconds . Voltage should decrease to 0.2 volt or less and remain there.
4. Create a rich condition for 10 seconds. Voltage should increase to 0.8 volt or more and remain there.
5. If sensor fails to vary according to the test steps above, the sensor is defective.
6. Connect sensor and clear codes.

If it passes this test then you will have to look at the wiring and a possible bad PCM. There is only one HO2S in the system.