Hi all. My 2002 Ram 1500 with 360/auto has a code P0420 at left bank. Does this mean that small cat convt looking thing at the drivers side head pipe is bad, or is it possibly the 02 sensor? No 02 codes showing but looking for some advice prior to spending money on wrong parts. Btw the truck has all the original 02 sensors/cats and has 286,000 miles so I guess I got my monies worth! Thanks for any and all replies.

Way over time to replace all the o2 sensors. 60k I believe is the limit? Might notice a fuel mileage increase by doing that.

with your scan tool read the o2s.. if they look good possibly the cat..if thats the only code.. if you have other codes fix them first..

and if it is the o2 is bad I would use mopar o2 (NTK/NGK) and not bosh o2s.

also if you do change the 02s and the codes come back, test the back psi in front of the cat and compare it to the psi in the rear of the of the cat.. simple to do.. you can find the procedure on you tube

with that kind of mileage, that cat is almost certainly wore out. Would not be a bad time to replace the 02's , based on mileage.

Thanks for the replies. Funny thing that additional codes got mentioned because there were two others, P0455(EVAP leak,large) & P0442(EVAP leak, small). I have since found out that the gas cap is missing(gotta love your kids) and would assume that is why those codes are showing. I will install a new cap and let you know how it goes. Is it possible that a missing gas cap would be responsible for the cat conv code (P0420) showing up or is that wishful thinking?

Put a new cat on it, if you do o2's first I think it's a waist of money.

I think you are probably right. What's the deal with the cats? There appears to be three of them. A small looking one on each head pipe and a bigger more normal looking one under the cab.

Under $120 to replace 4 o2 sensors from Rock Auto. Likely over $600 to replace pre-cats and cat, certainly over $250 for the rear cat only. Seems I'd start cheap and move forward from there.

That code means the post cat sensor isn't seeing enough difference between the pre cat sensor. Sure, it might be a bad cat but at the same time if the sensor that is 6x over it's expected lifespan is at fault, why spend the extra cheddar you don't need to? If you put cats in, would you leave the existing o2 sensors? Probably not.

Incidentally, the 99 Dakota drivetrain service manual starts with clearing codes, checking for exhaust leaks and age of o2 sensors.

also make sure u dont have an exhaust leak n front of the cat.. that will set the code.. 02s are the cheapest fix.. but if your wrong thats 120 u wasted... walker cats arent to bad priced too..

If your scanner can read live datastream look at what the O2 sensors are doing, if the up-stream O2 is fluctuateing real good then it is working and you need a cat, if it is not fluctuateing very much or at all then replace it before doing the cat.

A dirty upstream sensor can overwhelm the cat and make it apear bad.

A trick I learned recently was that if a guy cuts out the cat and replaces it with a pipe and runs a new upstream O2 and an old down stream O2 they will rear different even though there is no cat and keep the light off, At least on a 97 ram 2500 with a 360

Don't mean to flog a dead horse, but same deal here. '01 Durango R/T. Original O2 sensors, but cat is semi-new high flow 3"in/out Magnaflow cat. It was put on about 3-4 years ago when the headers and 3" system all the way to the back was put on. Been running great ever since. Decent (for a Durango) fuel economy. Total mileage around 95,000. New plugs and wires around 60K.
Can't remember if I replaced the PCV or not at the time, so doing that this weekend.
Also, Fastman TB and K&N FIPK kit on it too, if that matters. But they've been on since 50K with no problems. Filter could use a cleaning.
Been using a lot of Safeway gas (premium), so maybe all these factors could cause the code to get set?

Dealership says only Bosch sensors are available. Is he pulling my chain, or what?
As always,
TIA.
-Marq

po420 means the rear o2 is reading very similar or the same as the front o2 in a 2 o2 system. on a 4 o2 system its just the one bank. id say its the cat...ive done all the diag on a 01 in the last week, even did mopars recomend if the downstream is new and upstream is old replace it, code came back...so most likely its the cat, and while you have the cat out replace the 02s, you need to take them out anyway to weld in the cat or replace the headpipe. I would only run a mopar 02 the aftermarket parts are crap. also an aftermarket cat will meet the spec provided by the manufacturer, it will be on the bottom end of the scale tho. usually aftermarket cats dont last nearly as long as an oe cat, they dont use the same amounts of precious metals...which is why if you recycle an aftermarket cat you get 10 bucks an oe cat you get 75.

We installed probally a thousand Bosch O2s at work over the years.Never had a problem with them.Now the spark plugs they make are a different story.The new O2s made by Delphi are also top quality.I believe mopar gets theirs from Delphi as only a few companies make them.Most OE stuff is outsourced.Rocky

RockAuto's got Wells, but not Bosch. Had a Wells ECU on the RR for over 25 years with no problems.
I'd get the Bosch ones from the dealer, but they want $100 apiece for 'em. The Wells units at RA are $60 apiece. Anyone used the Wells units?

Sorry, spoke too soon. Searched by part number instead of vehicle,year,model. Bosch in stock.

Hey all, I just saw this thread and figured I'd update everyone. I replaced all 4 o2 sensors (NGK irrc) back in May and that fixed it . Passed smog with no troubles. Today it has a hair under 300,000 miles, and is still running all the factory original cats! The motor is untouched (the valve covers have never even been off) as is the trans... which just recently gave me a little trouble, but thanks to some guidence from you guys here on Moparts I fixed it by just changing the trans sensor and pressure solenoid . I just got lucky I supposed... well that, and Mopar built me one great truck .

Glad to hear it. Ordered two O2 sensors and a PCV valve off RockAuto yesterday. They were due anyway. Running some fuel system cleaner through with this tank. We'll see what's what in a couple of weeks after a tank or two.