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sprinkles08, ASE Certified Technician
Category: Jeep
Satisfied Customers: 22986
Experience:  Chrysler/Dodge/Jeep Master Certified, ASE Master and Advanced Certified, Trans and Hybrid Specialist
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I found fault code p0420 with my reader..catalytic..O2 sensors

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I found fault code p0420 with my reader. This the catalytic efficiency code. Would the problem most likely be one of th O2 sensors? 2003 Liberty with 103000 miles

Hello and welcome to Justanswer!


This code is for the left bank catalytic converter's efficiency. The converter monitors work by monitoring the switch rate between the upstream and downstream oxygen sensors. (upstream is before the cat, downstream is after) The engine controller knows that if the converter is working the downstream should switch from rich to center to lean much slower than the upstream. When the downstream sensor is switching at close to the rate of the upstream sensor it sets a failure code.


You've got one of two things going on. You either have a cat that has actually failed, or the upstream sensor on the left bank is getting lazy. As the sensors age they will start to naturally switch slower. At a certain point they will eventually set a code, but it's not at all unheard of for it to set a converter failure code before setting a code for being lazy.


If you have access to a full capability scan tool monitor the switch rates of the sensors. If you see the downstream is switching quickly you know the cat isn't working and will ned to be replaced. If the upstream isn't switching quickly, replace it with a Mopar sensor. If you don't have access to a scan tool I would start by replacing the left upstream sensor. It's far cheaper than a converter and a good start. It does happen once in a while on these, but converter failures on the Liberty aren't that common.

Customer: replied 7 years ago.

at what point of operation should the cat be causing a switch from rich to center to lean...just trying understand that if the downstream O2 sensor is switching slowly relative to upstream O2 sensor it means the cat is working. I understand that if both are switching equally, it is as though no cat exists. I have a live monitor code scanner that I just got. I can try to use this with the car running I would imagine to see if I can monitor the O2 sensors.

The oxygen sensors will be constantly switching back and forth quickly while the engine is running. If you do have a true scan tool that can monitor actual data this is a great chance to use it. Watch both the left (1/1) upstream sensor and the right upstream (2/1) and see if they switch at about the same rate. Then compare the downstream sensors (left - 1/2, right is 2/2). The downstream sensors should be switching slowly, loping along, while the upstreams should be switching quickly.

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