Hey guys,

For the last year or so, this has been an ongoing problem so I thought I'd see what your thoughts are.

Back in mid 2016, check engine light came on and code P0420 was showing by itself. Bank 1 cat efficiency code. truck had just over 110k on it then.

I use an independent mechanic, they are JLR specialists, have been around forever etc. Shop is always full of early XK cars, XKEs, and the newer stuff too. They have a good reputation and I trust them. They said to replace the cat, so I did. They used a MagnaFlow one, said they have put it in quite a few LR3s with no issues and was cheaper than OEM. This was around July 2016

Well, February 2017 comes around and P0420 again. Shop said they never had a MagnaFlow cat fail on them before, but they warrantied it and put another one in no charge around March 2017.

Here we are in December 2017, and, yep you guessed it: P0420 again. I am now on the third catalytic converter for Bank 1 including the original, and looking at a 4th. I'm sure this one would be covered under warranty again - but I think there must be something else going on here - I have never heard of any vehicle chewing up 4 catalytic converters on the same bank in 120k miles/9 years. The truck is meticulously serviced, synthetic oil every 5k miles, regular fluid changes - it gets everything it needs. It has been sitting more than it used to, as it is no longer my everyday car.

Thoughts?

 

I get the P0420 code triggering my CEL every now and then but it clears itself after a few days at most. I read somewhere that bad fuel or even the change to a winter fuel mix can trigger the cats but that they reset and clear themselves. At least that has been my experience.

 

P0420 is NOT an automatic 'bad Cat' code and the cat should not be changed as the first, but blind, attempt to fix it. The code is showing that there is a problem associated with something in the exhaust system and should be checked out.

The code could be raised by a faulty rear O2 sensor or a leaking exhaust.

The first check should be to examine the exhaust pipes carefully for damage and possible leaks especially between the 2 O2 sensors. Unmetered air getting into the mix at this point won't be sensed by the front sensor but the rear one will pick it up and signal a fault.

I found such a leak in my own exhaust by doing a smoke test on the system. It showed up a leak around the boss of one of the front O2 sensors which needed to be rewelded.

If the system is leak free, then, the rear O2 sensors should be the next suspect. Check the wiring and connections and if all's ok there they should be tested using a scan tool and/or oscilloscope.

Only if all these checks show no problems should the Cat be suspected. As you've already had a new Cat fitted relatively recently and the problem has recurred I'd be more certain than ever that there's a problem somewhere other than the Cat.

Even if the cat has gone bad the question has to be asked, Why? New Cats don't just fail, there has to be an underlying cause. The worst problem that can cause a Cat to fail is unburnt fuel which ignites in the Cat. This can be caused by an undiagnosed fault, for instance an O2 sensor signalling the ECM into a rich mix situation. Goodness knows, even a leaking injector could cause unburnt fuel to reach the Cats.

Your garage may be good on LRs and Jags, but they should not just jump in with fixes without going through the diagnostics and check sequences. Cats are just too expensive, no matter the make, whereas a faulty O2 sensor or leaky exhaust is a lot cheaper to fix.

 

My previous 2007 had this problem for nearly a year. I found several generic mechanical web sites that explain the P0420 problem and all say the cat is that last place to look.

I agree with Alphmale, exhaust should be looked at first. A hole in the exhaust in front of, or between the 02 sensors will create a problem. I busted the the left one on a rock and had this code. It went away after I had it welded up. I then got it on bank 1 (right side) a rear later.

Since the 02 sensors help control the fuel management, you need to make sure you have a clean engine air filter followed by a clean and working MAP sensor then a clean throttle body. After this look for leaks in the intake hoses after the MAP (you are trying to find any unmetered air)

Remember the computer changes fuel levels based upon 02 sensor readings. If you have unmetered air it most likely could create a lean mixture even as the computer tries to add more fuel to make up for the extra air.:

which in turn burns cleaner;
which in turn does not allow the cat to heat up;
which in turn causes the efficiency to drop below a predetermined difference between the front and rear 02 sensors;
Which in turn trips the CEL.

I did all of the above short of replacing the cat. By the way which bank is it? Bank 1 (right side of vehicle is most common and the more expensive cat)

I finally placed an 02 sensor spacer available at that the local parts store on the right rear sensor which pulls the sensor back slightly from the exhaust flow and is just enough to stop the code from coming back every few months.

If it is the left, you may have a difficult time reaching the 02 sensors without removing the transmission cross brace and front drive shaft. it is doable but you need someone with thin hands.

But then again, I could be wrong about the whole thing.

I hope my ramblings helped

Jeff

 

I had a bit of a different situation since my engine ingested a serious amount of oil when my PCV failed (and took out a bearing) but I got the same codes (intermittently on both sides) after I had the engine replaced. I figured it was the catalytic converters since the shop said they might have been damaged, so I tried a spacer in the rear O2 sensor on both sides. This fixed it temporarily, but the same codes came back and wouldn't go away, especially on the passenger side. I bought a new OEM passenger side (bank 1) converter ($$$) and the code STILL came back. So I got two new Bosch O2 sensors (front and rear) for bank 1 and the code has been fixed since. They were far cheaper than OEM and work fine (and probably are the same part).

My bank 2 still throws a code for low signal or something similar every 6 months or so because it gets confused by the spacer in the rear O2 (it's still there on that side) and clearing the code doesn't work. I have to use my IID tool to reset all sensors and learning. I should probably get another pair of O2 sensors installed on that side as well, but I'm dreading finding out that the O2 sensors AND the converter are actually bad.

 

Thanks for the replies everyone. I'm going to print this thread and show it to my mechanic. I'm assuming they would have checked for exhaust leaks and a bad 02 sensor, especially because the warranty replacement cat came out of their pocket as far as I know - and those are fairly easy things to check.

It is Bank 1 for the record.

I'm bringing it in next week, so hopefully I'll have an update for you guys then.

Thanks for the wisdom!

 

ChicagoRover, never assume anything, because it just make an '***' out of 'U' and 'Me' or as in this case 'Them'.

Some times a shop will just jump to a conclusion, without going through the proper diagnostic sequences, because that's what worked before. But, 'what worked before' is not a proper diagnosis, it rings of a lackadaisical, through-other attitude.