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P0420 but only when driving on dirt roads

Hello everyone. LOOOONG time reader/lurker but this is my first time posting. I want to start with a huge thank you to all the regular posters here. You have helped me more times than I can count.

2002 4Runner 4x4, 180K miles. I bought this car about a year ago and don’t know anything about its history before that but it seems to be in good shape.

About every month or two I take a trip that includes driving a few miles on a dirt road. It is a graded road in pretty decent shape. What has happened about 6 times during the year is that the CEL comes on while I am driving on the dirt road. The car continues to run well. When I get home and hook up my code reader the code is always P0420 Catalyst efficiency below threshold.

To save everyone the trouble of looking that up, this code indicates that either the upstream or downstream A/F Ratio sensor is bad. If both sensors test good then it is probably a bad cat.

What is different here is that after I clear the code, the CEL stays off as long as I stay on paved roads. So to summarize, I don’t get the code every time I drive on a dirt road but every time the CEL has come on it is while I was on a dirt road.

So it seems that something about driving on a dirt road causes the CEL. Two obvious things you get on a dirt road are dust and vibration. Does anyone have any thoughts about what could be causing this? I did a visual inspection of the whole exhaust but did not see anything obvious such as cracks or leaks.

And of course I could live with this if I have to since it comes up under such limited circumstances. I just like to keep my cars running properly.


Check this out . . . I believe you have the 5VZ-FE engine

There’s a loose connection to the rear sensor.
The constant vibration of the dirt road causes the loose connection to rapidly make and break, which the computer interprets as the rear sensor changing state.

Yes, I do have the 5VZ-FE engine. I don’t have anything to read the sensor output but even if I did I don’t know if running that diagnostic procedure would help. I suspect the sensors would probably be working correctly while the car is stationary. It seems like I would have to rig up a way to take the readings while I was driving on the dirt road. Maybe I could hang out the door while my wife drives! (Note to the humor impaired - that was a joke).

The loose connection seems like a good possibility. I have a multimeter so I can check for discontinuity while I move the wires around.

Thanks for the responses. You guys are great.



Can I borrow your car. My wife as been a pain lately and I thought that if I trained her to read a multimeter and hang out the door on a bumpy dirt road>>>>>>>>>>>>>>>>>>>

What the heck . . . ?!

You have that backwards. Begging is what husbands do to their wives; while training is what wives do to their husbands. :wink: