How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask sprinkles08 Your Own Question
sprinkles08, ASE Certified Technician
Category: Jeep
Satisfied Customers: 21289
Experience:  Chrysler/Dodge/Jeep Master Certified, ASE Master and Advanced Certified, Trans and Hybrid Specialist
Type Your Jeep Question Here...
sprinkles08 is online now
A new question is answered every 9 seconds

I found fault code p0420 with my reader..catalytic..O2 sensors

This answer was rated:

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 6 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.

sprinkles08, ASE Certified Technician
Category: Jeep
Satisfied Customers: 21289
Experience: Chrysler/Dodge/Jeep Master Certified, ASE Master and Advanced Certified, Trans and Hybrid Specialist
sprinkles08 and 5 other Jeep Specialists are ready to help you

JustAnswer in the News:

Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.

What Customers are Saying:

  • Thank you so much for your answer. My initial estimate was $325; I got it done for $100 - part and labor. Lynn B. Ormond Beach, FL
< Previous | Next >
  • Thank you so much for your answer. My initial estimate was $325; I got it done for $100 - part and labor. Lynn B. Ormond Beach, FL
  • I was skeptical at first but thought I'd give it a try. I was pleasantly surprised. Your site actually is a quality service that I would recommend to others in a heartbeat. Stan Sayre, PA
  • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C. Freshfield, Liverpool, UK
  • This expert is wonderful. They truly know what they are talking about, and they actually care about you. They really helped put my nerves at ease. Thank you so much!!!! Alex Los Angeles, CA
  • Thank you for all your help. It is nice to know that this service is here for people like myself, who need answers fast and are not sure who to consult. GP Hesperia, CA
  • I couldn't be more satisfied! This is the site I will always come to when I need a second opinion. Justin Kernersville, NC
  • Just let me say that this encounter has been entirely professional and most helpful. I liked that I could ask additional questions and get answered in a very short turn around. Esther Woodstock, NY

Meet The Experts:

  • Chris


    Shop Foreman

    Satisfied Customers:

    Jeep Certified Master Tech
< Last | Next >
  • Chris's Avatar


    Shop Foreman

    Satisfied Customers:

    Jeep Certified Master Tech
  • Mark's Avatar


    22 Year Factory trained technician

    Satisfied Customers:

    Jeep factory training........25 years in the industry.
  • Dave's Avatar


    Jeep Master Tech

    Satisfied Customers:

    Chrysler Triple Master, ASE master Tech AA degree in Auto.
  • moparfl's Avatar


    ASE Certified Technician

    Satisfied Customers:

    ASE Master Tech Chrysler Dodge Jeep Master Tech
  • Tim Mohr's Avatar

    Tim Mohr


    Satisfied Customers:

  • Ron Z.'s Avatar

    Ron Z.

    - Jeep Tech -

    Satisfied Customers:

    18+yrs experience. State Inspector & Jeep Diagnostics/Repair
  • jeepmopar76's Avatar


    Jeep/Chrysler Triple Master Tech

    Satisfied Customers:

    32yr ASE Master Tech, 9yr Chrysler Master Tech, 19 yr Jeep Dealership service including AMC/Eagle