CEL code P0420 O2 sensor How to pinpoint the bad one? Help please. - JeepForum.com
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Unread 08-18-2014, 04:26 PM   #1
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2004 LJ Wrangler 
Join Date: Dec 2013
Location: Oak Forest, Illinois
Posts: 18
CEL code P0420 O2 sensor How to pinpoint the bad one? Help please.

I have an 04 LJ. My check engine light has been coming on and off for the last 7-8 months. Its been the same code each time. P0420. Basically it says its a bad cat, engine mechanical problem, or bad o2 sensors. Im assuming that since the light is turing on and off every couple months, its an O2 sensor. cause the engine runs and sounds fine. just the noisy lifter that is notorious with the 4.0. Basically I was wondering if there is any way to pinpoint which O2 sensor is bad so Im not replacing 45 bucks or more worth of O2 sensors. Also any input on what it could be other than an O2 sensor would be appreciated. I leave for school on wednesday. Stuff always seems to go wrong right before I leave for school or home.

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Unread 08-18-2014, 04:45 PM   #2
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2000 XJ Cherokee 
Join Date: Apr 2002
Location: out in the garage - Minneapolis, Minnesota
Posts: 21,415
The Check Engine Light is ON, the code tells you which one is bad, or you can test them all. The Google knows everything, all you have to do is search and read.

P0420 JEEP Catalyst System Efficiency Bank 1
- Engine Light ON

Possible causes
- Three way catalyst converter (the front mini-cats)
- Exhaust manifold leak
- Fuel injectors may be faulty
- Fuel injector leaks
- Spark plugs may be faulty

Tech notes
As the code description implies the P0420 code means that the vehicle's control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem.

The Engine Control Module (ECM) monitors the switching frequency ratio of heated oxygen sensors 1 (front O2 sensor) and heated oxygen sensors 2 (rear O2 sensor).A three way catalyst converter (Manifold) with high oxygen storage capacity will indicate a low switching frequency of heated oxygen sensor 2. As oxygen storage capacity decreases, the heated oxygen sensor 2 switching frequency will increase. When the frequency ratio of heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst malfunction is diagnosed.

When is the code detected?
- Three way catalytic converter Bank 1 does not operate properly.
- Three way catalyst converter Bank 1 does not have enough oxygen storage capacity.

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