I changed the upstream sensor about two months ago and the downstream (cat) one about 2 years ago. Yet I still get the MIL O2 sensor fault. Should I replace the downstream one again? Is there any way to bench test the sensors?
Mel P.
96 Cherokee
The fault code doesn't necessarily mean the sensor is bad. Just means that the 'puter is seeing info from the sensor that is out of expected parameters. For example, if you've got a bad plug wire or a fouled plug, the 'puter would see excessive oxygen in the exhaust stream and throw the code for the sensor. If you've changed both sensors and haven't torn the harness off one of 'em(like I did this winter)you might check elsewhere for the problem.
Besides it being something else putting the O2 out of range, it is at the age where all the plugs and sockets for the computer related sensors could benefit from a good clean. I normally use a proper electronic contact cleaner and then dielectric grease on them.
I used to do our Cherokee all around the engine compartment every couple years or every time the TPS got cranky. The CPS connection is so bad, Chrysler even recommends soldering the sucker on some years of them so there is no connection to go bad.
When they get old, chrysler connections like to corrode and the slightest bit of corrosion will interfere with the really low powered computer signals trying to pass through the contact. With higher powered or analog systems, a power crackle doesn't matter, with low powered digital, a crackle is a signal...
Just a thought for you, a clean is cheap and relatively easy and if it works you are good to go.
Mel P. wrote:
Doesn't the DTC tell you which O2 sensor it is that is at fault?
You can't bench check them without an oxy-acetylene torch and even then it's not really a test. You can watch them on the vehicle if you have an O-scope. The peak and valley plus rise/fall times will show if the sensor is getting old. I seem to have lost my scope captures for the O2 sensor. :(
If you have the I6, check just forward of the exhaust side motor mount. The sensor wires run through a metal band that tends to wear the cable over time. I had to splice mine because it pinched and melted the cable.
Your best bet is to go get the cheaper $35-45 OBDII scantool so you can read all the codes on your ZJ. Your year didn't come with very good diagnostic display.
How old is the CAT? A failing or bad CAT can cause the O2 sensors to also cause a fault.
thanks, looks like I'll be replacing plugs wire etc.
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