96GrandCherocheap

Hello, I have a 96' Grand Cherokee Laredo 4.0L, and just recently had a new crank sensor installed. The car has been running a lot better than it did before but now the check engine light is on. I went to Autozone and had them run a diagnostic. They came up with the code P0420, which was defined as this:

Catalyst efficiency low-bank 1
Explanation:
The oxygen sensors monitor the Catalytic Converters ability to store oxygen.
Probable cause:
1. Catalytic converter defective (failure possibly due to #2, 3, or 4)
2. Engine misfire or running condition
3. Large vacuum leak
4. Engine oil leakage into exhaust-valve guide seals, piston rings

I asked the lady doing the diagnostic at Autozone about changing out the oxygen sensors and she said that usually won't solve the problem but i hear a lot of people doing that first before changing the cat. I do have a mechanic but have been giving him all my money on this Jeep. Any advice would be helpful. Thank you!

gully369

When my O2 sensor at the catalytic converter was bad (98 ZJ) I had code P0138 O2 circuit high volt (Bank 1, Sensor 2). Wish I could be more help.

96GrandCherocheap

Hey - thanks for the response. I'm beginning to think that it may be my converter now. Since the time I posted my first message my car has just been acting up pretty bad. I drove my car to my friends, who lives 20mins away (mainly on freeway) with no signs or symptoms. My car sat there for over 2 hours and then I go to leave. I start the car and it immediately stalls, I try and restart the car, it stays on for one second and then stalls again. I go to start it a third time and it catches but the rpm's are moving up and down pretty quickly from 1300-200rpm while in park. You can imagine that it feels like it is about to stall out again but after less than a minute of doing this, it goes to a steady idle of where it should be but still slightly rough. I wait another minute and then head on the 20min drive home. I'm on the freeway and 2 minutes before my exit the rpm go down as well as my speed. I could not get over 50mph. A minute before this happens, it felt like there was a pretty significant misfire or backfire. I get off my exit and, while not pressing on the gas, the car doesn't loose power but the rpm are at zero and the battery gauge almost sinks to dead negative. I pressed on the gas and you could feel the car kick back to normal. Anyone experience this same problem with the code from above?

athenscj5

Any resolution to your P0420 code? I am showing the same code and I have a 1.5 year old magnaflow catalytic converter. Hoping it isn't the conveter. Juat curious what you figured out.

gmv93

you can check your sensors with two digital multimeters. you need to find wiring order of sensors then you will connect each multimeter to each sensor and it will measure voltage. do not connect to ground on wires.use car chasis. you will need to see different sequence of changes on voltage read. If readings are very similar or exactly same, your converter is dead. engine must be warmed up to measure voltage.

willswheels

i have a 1996 Grand with a 4.0 that has a check engine light showing the po420 code. it has an ALL NEW (from eshaust manifold to tailpibe) factory exhaust. according to the VIN the jeep was repaired BY A DEALER per the 2007 po420 recall (new cat & computer reflash) in 2007. WTH?????? DEALER responds with a "bring it in 4 a friendly $110 diagnostic" but given the circumstances I truly believe the dealer DOES NOT KNOW HOW TO CORRECT THIS PROBLEM. do any of u?

Pappy C

Preface Note: Used "search" function extensively. This thread is most relevant, but still no answers. ready to ask the question:


Alright guys...I'm at my wit's end. I have a 98 XJ, 4.0L, AT.
- Had a crack in the muffler, so I had a new magnaflow welded on.
- While it was off, had them inspect the Cat (also a magnaflow), at was told it was perfectly clear.
- CEL came on and gave me a 138 and a 420 code.
- Needed some new kit, so I proceeded to replace:
- O2 Sensors (upstream and downstream)
- all plugs
- pulled exhaust manifold, welded crack
- replaced w/new manifold gasket
- Already replaced CPK and TPS


Cleared the old codes, and CEL came back on. STILL reading 0420 & 0138. Please help before I go well and truly insane...


Cheers,
Pappy

Bustedback

The 420 code means the catalytic converter is at 95% or less efficiency. You need a converter. Not uncommon for the cat to fail after seven years. Have the diagnostic done and see if they find anything that may have contributed to the failure of the cat.

Pappy C

Thanks for the speedy response, Bustedback. Everybody can disregard. I figured out why this "problem" doesn't exist when using the search function; I didn't disconnect the battery long enough to clear the codes. :/ disconnected the battery longer this time, re-ran the scanner and lo and behold: 0 codes. Sorry for the false alarm, thanks for humoring me, and have a great one.


Cheers,
Pappy

Gerald L. King II

As it's been awhile since this was updated, I wanted to do that since we are facing the same issue with our '99 Grand Cherokee.
Don't jump to the MOST expensive part first, there are a couple of cheaper alternatives that you can try. (We just purchased this Jeep, our 3rd, and one of the selling points was that the Converter was recently replaced). Here is a good explanation of the P0420 code:

https://www.autocodes.com/p0420_jeep.html

Hope this helps!!!

PS: my code reader says it's an O2 Sensor, Bank 1, Sensor 1 so we'll start there.

UPDATE: First, I was wrong about the Catalytic Converter being replaced.
Second, we replaced the Bank 1 Sensor 1 and cleared the errors. Following the recommendation from autocodes, we purchased come Lucas engine additive and filled up with Premium from Chevron. We drove around for almost an hour and a half, close to 75 miles, which allowed several cycles to run for the Computer/Sensors and the check engine light has not come back on. Immediately after replacing the Sensor and starting up, my daughter says she can smell a distinct difference. I could still smell that it was running rich, BUT it was not NEARLY as bad as before. We'll go through the bottle of additive before we decide where to go from here.
Crossing our fingers.

97grand4.0

I will weigh in here with a recent experience with PO420 that I fixed for $13 and a 10mm socket, on a 2010 Honda CRV 4 cyl. with 65k miles...otherwise not a single problem. Code popped up out of nowhere. Googling showed this guy who was saying dont go ripping apart your exhaust or replacing expensive converter and 02 sensors. If you have a plastic intake, its likely the intake has warped or is cracked and or the little intake O rings are leaking. Huh, I said to myself, it does have a plastic intake.
When I take a close look, someone had replaced the air filter and left both of the big worm hose clamps loose. Tightened them up and guess what. Reason being, the MAF sensor is AHEAD of the air filter, it's right at the front of the plastic snorkel feeding the air filter. Hence, the engine was pulling in air AFTER the MAF...unmetered air into the engine. Not a vacuum leak, but still, more air than the computer was calculating when it did fuel trims.
I will add that before I found that, I put a can of "Gauranteed to Pass" snake oil in a full tank of gas, guaranteed to clear a 420 code or double your money back. It supposedly burns out deposits from a cloggy converter. So it was very near the end of the tank of gas when I checked and all the monitors had run from the last time I cleared codes, and no MIL.
That was 2 weeks ago and the car has gone hundreds of miles with no return of the PO420. as someone mentioned, the code is not saying the converter is bad, only that there is less than 95% efficiency, meaning t here is more than 5% oxygen present in the exhaust after the converter at the O2 sensor. It senses oxygen, and the job of the converter is to take oxygen OUT of the exhaust. So a very good way to monitor the converter efficiency, but not necessarily points to the converter if it senses too much oxygen. Hope that helps.