Is my Oxygen Sensor dead?

Question

I have a 2001 Subaru Forester, 120,000 miles. Recently I've been getting the check engine light, so I bought some OBD software and found that the trouble code was low catalytic converter efficiency.

But, when I look at the O2 sensors using the software, one of them consistently shows 0.0 volts, even after driving.

The other shows varying voltages depending on what the car is doing.

Everything I've read leads me to believe that the sensor showing 0.0 volts (Bank 1 Sensor 1) is bad, and that's likely causing either the mixture to be too rich (if sensor one is the upstream sensor), or for the OBD computer to think the cat is bad (if sensor 1 is the downstream sensor). Before I go ask a shop to replace the sensor, I'd just like a sanity check that 0.0 volts = bad sensor.

Answer 1

Well generally speaking you should replace those items every so often, like 60,000 miles.. so id say youre due.

as to the voltage, providing that you are mesuring the correct leads.. then yes, the sensor is bad.

There are usually 4 wires to a O2 Sensor, two are for heating, and 2 are for signal. If you have the signal wires, they should read 0 volts cold. With the engine warm at idle, they should read around 100millivolts (0.1) and with the engine running at 1700RPM you should see anywhere between 300 - 500 Millivolts.

If you have zero.. get some new ones!

Answer 2

Find out what voltage the sensor should be sending, but it sounds to me like it's deader than nails. Especially if it doesn't change warm or cold.

Your local auto parts store should be able to provide that info. Now if I remember correctly, most O2 sensors should run from 0.5 to 0.7 volts, but it's been years several years since I took a measurement. If you have a friend who will let you look at their OBD II vehicle, you can cross check. All O2 sensors operate the same way and send the same voltage signal to the cars computer.

Hope this helps.