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P04020 code

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In trying to diagnose this, let me add that the was a muffler leak right behind the cat which was replaced but the code did not stay away when cleared. Also, the car seemed to be running fine. Edited by dvto2
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So i found some stuff on the advanced auto website for testing the o2 sensors. For the front sensor it says to measure between B136 terminal 13 and terminal 1 and B136 terminal 22 and terminal 2, replacing the sensor if the resistance is below 1 ohm. For the rear sensor it says to measure between terminal 4 and the engine ground, replacing the sensor if the voltage is between .2 and .5 volts. But there are a lot of models in the description and it's not always clear what description goes with what model. Do these descriptions sound accurate?
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I think for the front sensor notes, that's terminals 13 and 22 on B136 to 1 and 2 at the front O2 sensor, so you're essentially assuming that if you have continuity in the signal wires, the sensor must be bad. The rear O2 sensor diagnostic is measured on the car-side harness with the sensor unplugged, and that verifies that everything on the car side is in good shape. P0420 is for the rear sensor, though, and since the car runs fine, it's a safe bet that it's the sensor that's bad and not the cat.


From my notes on diagnosing the sensor on my Baja (everything has matched up with what you've said so far, and I'd have no reason to think it'd be different), it looks like you should measure less than 30 ohms resistance between pins 1 and 2 (heater circuit) at the sensor. Terminal 4 at the sensor is the signal, and according to the FSM, that should be somewhere in the 0-0.9V range at idle, and 0V with the key off. Terminal 3 is sensor ground, and that should obviously show 0V in both states.


I'd unplug the sensor, key on, then crawl under the car and check voltage from car-side pin 4 to ground and check resistance across sensor-side pins 1 and 2. Assuming that all points to a bad sensor, spray some PB Blaster on there daily between now and when your replacement sensor shows up, and the old one will come out much more easily. If you're the DIY type and you're comfortable cutting and pasting wires, you can use this sensor and reuse the existing sensor's connector to save some cash, otherwise the plug-in version (which should be this one) should do just fine.

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