akoop

The check engine light came on this morning on the way to work in my 2010 Wrangler Unlimited. Stopped at AutoZone on my way home and they pulled the code, it is P0420 and says "Catalyst System Efficiency Below Threshold Bank 1". The tech said I would probably need a new catalytic converter but that seems extreme. Is there a way to test the O2 sensors? Can someone tell me what Bank 1 means?

Thanks,
Alan

ronjenx

Bank 1 means the right side of the engine.

There is a lot to look at before the cat is condemned. In the list below, you'll notice the cat is the last item.

Possible Causes
O2 RETURN UPSTREAM CIRCUIT SHORTED TO GROUND
O2 RETURN DOWNSTREAM CIRCUIT SHORTED TO GROUND
O2 RETURN UPSTREAM CIRCUIT OPEN
O2 RETURN DOWNSTREAM CIRCUIT OPEN
EXHAUST LEAK
ENGINE MECHANICAL CONDITION
AGING O2 SENSOR
CATALYTIC CONVERTER

NOTE: A new rear O2 Sensor along with an aging front O2 Sensor may cause the DTC to set. Review the repair history of the vehicle before continuing.
NOTE: If an O2 Sensor DTC set along with the Catalytic Converter Efficiency DTC diagnose the O2 Sensor DTC(s) before continuing.
NOTE: Check for contaminants that may have damaged the O2 Sensor and Catalytic Converter: contaminated fuel, unapproved silicone, oil and coolant, repair necessary.

akoop

I figured that the CAT would be one of the last items to replace. I have had this Jeep since day 1 and I know none of the O2 sensors have ever been changed. The Jeep is a 2010 with just over 150K miles on it. My guess is it is one of the O2 sensors. The jeep seems to running fine, no rough idle, etc. Is there any way to test the O2 sensors or should I just replace?

resharp001

Given the age, you're just rollin' up on replacement time IMO. The sensor is the most likely culprit by far.

If the autozone workers in your area are anything like mine, "tech" is a VERY loose term. It wouldn't surprise me at all to hear one of them say "you probably need a new cat". Lot of them are just barely good enough to change your battery and replace your wipers.....no offense to anyone here that might work at autozone.

mr72

IME working on many different cars over the years a P0420 for a car with >100K miles on it pretty much always requires a new cat to fix. This is especially true if you ever had a misfire due to ignition failure (even intermittent).

While it's possible for other things to cause this code, at that mileage it's a 95% probability it's the cat. As routine maintenance I would replace both cats and all O2 sensors at once given that mileage, that is if you intend to keep the Jeep another couple of years. If you just want the code to go away as cheaply as possible so you can sell/trade the Jeep then you will be likely wasting time and money on O2 sensors if you don't also replace the cat.

resharp001

That's strange....I've had exact opposite experience. I've never had to replace a cat....anytime I've ever seen that code on one of mine in the 100k+ range, it's just been o2 sensor, and I've replaced my fair share of em.

Yamaha90

I had this code pop up in September. And after that I noticed the Jeep running more labored. Even on regular stretches of road it would normally be fine on i noticed higher RPM for longer periods before shifting. Me personally I looked at the readings of the O2 sensors on my Superchip and the voltage was fine so they weren't shorting out, and the bank one results were different then bank 2.. So it may not be an O2 sensor for me, but if the code comes back with NEW cats on then I'll replace the sensors as well since those aren't too bad to get to.

I finally bought a set of cats and will be installing this weekend. I got a set of EBay cats for less than 300 bucks, well aware that you get what you pay for but they have a 5 yr 50,000 warranty so I figured why not. Unpacked them yesterday and they actually look pretty good. I will be doing an install video for them as well via my page JMG Designs.

Emissions warranty is 8 years so you might want to have the dealer look at it if it is covered. If not then you can always go about your normal route of diagnosing the problem.

Yamaha90

r

Bank 1 is the side of Cylinder 1. Bank 2 is the side of Cylinder 2. On each side there are 2 O2 sensors, 1 upstream of the Cat, and 1 downstream of the cat. The downstream sensor will pick up the readings and throw the code that the Cat is not functioning well, allowing more gasses to pass then the "threshhold" will allow.

Mr.T

The way I test is to look at a graph of the O2 sensor voltages. It takes a scanner or other device that can graph the O2 sensor voltages, preferably more than one at a time, via the OBD port. When they fail, the response is either slow, failed high or low. Normal operation pre-cat is cycling slightly rich to slightly lean, and the rate depends on the RPM. The post cat O2 sensors should have a non-cycling mid scale voltage, cycling post cat indicates the cats aren't functioning well. The post cat sensors not only look for cat failure, they are also used to trim the F/A ratio slightly.

mr72

BTW this is what the P0420 means, it failed to match the expected cycle difference between the two O2 sensors.

If one O2 sensor is bad then it will report another code, O2 sensor voltage low (usually) or possibly high. It may also fire a P0420 which would indicate that the O2 sensor is the root cause.

Yep, but ordinarily this won't trigger a P0420 alone. You'd also get something like a P013x ...

A lot of carbon buildup on the O2 sensor (either pre- or post-) can cause a P0420 that's just the O2 sensor that needs replacement, but to get that much carbon buildup usually means it's been running very rich and running very rich will foul the cat pretty fast. And how do you get it to run rich when the upstream O2 sensor is not bad anyway? so basically to wind up wth a P0420 that's not a bad cat you have to have a dodgy upstream O2 sensor misbehaving for a short enough time that it runs rich and carbons up the downstream sensor but doesn't foul the cat, then in a magic interval of time you swap both O2 sensors before driving it in this state long enough to foul the cat. Who knows how long the magic interval is. Maybe 10 minutes, maybe 10 days. Not longer than a month of regular use that's for sure. So if you light up a P013x code and drive straight to Autozone, get a new sensor and swap it when you get home then you MIGHT get a P0420 on the way home that's not indicative of a bad cat and maybe you save the cat. But if you run it like this for a couple of weeks you're going to torch the cat and the downstream sensor too. At 150K all of them are old enough to warrant replacement of the whole enchilada.

The other thing that can cause an errant P0420 is if you have highly modified exhaust, like tube headers, larger primary pipe size at the collector, larger diameter high-flow cat, etc. This reduces the exhaust velocity which gives it more time to cool and results in low voltage reading especially on the downstream sensor, which will cause a P0420 that usually pops up when you get on low-rpm cruise for a few minutes. Used to happen on my Miata all the time, but that was with head-to-tail custom exhaust. But this is not even in the realm of reality for JKs. Nobody's out there swapping headers with big collectors and 3" primary exhaust tubing size that requires reprogramming the set voltage limits for the O2 sensor map.