# Code P0420 on '12 VW Routan with 130k miles from faulty oxygen (O2) sensor?



## bmlawren (Aug 29, 2020)

Hi all. I'm new to the forum and here's the short story...

I'm trying to figure out the potential cause of a code P0420 (bad catalyst code) which is popping up on my ODB scan tool before the oxygen sensor test completes. I suspect it's a bad oxygen sensor on Bank 1 (the right side) but I wanted to reach out to the forum to see if anyone can make sense of this. I can post specs from the readout (fuel trim, load, engine temp, run time, etc.), but having to understand an ECM and what it is doing and how data from the O2 sensor impact engine performance and why is all fairly new to me. Any suggestions before I either (a) spend $100 and put new oxygen sensors on the right side exhaust (bank 1) or (b) spend $800 and put a new catalytic converter and oxygen sensors on the right side exhaust would be appreciated.

The long version requires a little backstory on the van... in summer 2016, we bought a used 2012 Routan SEL and were happy with the minivan understanding it was a Dodge/Chrysler in VW clothing. Had taken many roads trips from GA to FL and back (at least 8-10) and never had an issue. It was our family car (two adults, four teens, and a dog). Then the drama started. In spring 2018, the TIPM went out... a week after the extended warranty expired. Begrudgingly, I had that replaced knowing the car would be completely inoperable without it. Then, in winter 2018, while en route to a funeral out of town, the check engine (MIL) lamp came on 300 miles from home. The driving behavior of the car was consistent with a misfire, so hoping for the best, we assumed it might be a dead spark plug / ignition coil (code P0306). Upon getting home (a really long, slow drive), we found that it was neither. Instead, a pair of rocker arms on the intake side of cylinder 6 had failed. There was also notable wear on the camshaft, to boot, but nothing the ECM couldn't handle as the rocker arm replacements seemed to do the trick.

Fast forward about a year to winter 2019... check engine (MIL) lamp comes on again (code P0304)... This time I had the dealer evaluate the cylinder head for the left side replacement so, while I got a new cylinder head out of the deal (adding in my purchase of two new camshafts, all rocker arms and lash valves)... this is really where the headache began. Within two miles of leaving the dealership, I heard it. The Pentastar click... almost as if someone had turned our engine into a set of maracas. Check engine light came back on (code P0303) so I drove straight back to the dealer since they had been the only ones to touch it for the better part of two months. Diagnosis? A dead lash valve (valve lifter) on the right side. Despite the misfire on cylinder 3, it turns out that the dead lifter was at cylinder 1 because this time, rather than throwing good money after bad at the dealership, I decided to do the right cylinder head fix myself (upside to working from home during a pandemic is that I have way more free time).

As with the left side cylinder head, we found excessive cam wear (a groove about 2mm deep) on the intake camshaft at both cylinders 1 and 3. The ECM on most cars is good... but not that good. As well, one of the lash valves had split in two (cylinder 1) and two of the rocker arms had lost nearly all of their needle bearings (cylinder 3). To make matters worse, one of the six-point (Torx-like) bolts in the camshaft caps had been stripped up top (as if someone had used a hex wrench to tighten itinstead of a starbit). My first thought... sabotage? Could the dealer tech's be this vindictive? After all, they had spent eight hours on the left cylinder head repair... rerouting all manner of wire harnesses in any which way and closing up without putting all the parts back in (the damper cushion that is supposed to rest between the left cylinder head and the upper air intake manifold was lost to the void).

Well, after coming down from that madness, I realized that a more likely scenario for the engine issues, especially given all of the associated problems with the left cylinder head before this, is that someone in our Routan's past (the crazy ex-owner?) had DIYed the rocker arm replacements and just didn't do it right. Given this scenario and the state of disarray in the right cylinder head, I was wholly concerned that maybe one of the valve springs, seals, retainers was shot too but it turns out (luckily) that's not the case. Hoping for the best, we pushed on and after fishing out all of the bearings, replacing the lash valves and rocker arms on both the intake and exhaust side, and putting a new intake camshaft into service... engine purrs like a kitten!

Of course, now that the misfire is cleared, I can finally fix all of the other things that were wrong with the vehicle but masked by this issue. First problem solved was the coolant thermostat. It died so I replaced it (pretty easy fix) which brings us full circle to the code P0420 (the real reason I started this thread).

I can post images of the shot camshafts, exploded rocker arms, and lash valve that literally looks like a broken nesting doll now in case anyone wants to see what their parts shouldn't look like. Still trying to understand why Chrysler would put $5 valve lifter which, when broken, can result in catastrophic engine failure thus requiring an eight hour shop time job to fix... but I guess that's why I'm not an engineer.

In any event, if anyone has any suggestions for what to check before I put new oxygen sensors on the car or just go ahead and get a new catalytic converter and oxygen sensors... I'm all ears. As much as I might sound like I'm complaining about our van, it really has been a neat puzzle. I just hope I'm picking up the last piece and putting into place for awhile. Thanks in advance for your attention, even if just for reading this far...

--bmlawren
2012 VW Routan SEL (2nd owner)
3.6L Pentastar VVT Flex
130k miles


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## Zambee500 (Jun 23, 2011)

I have the 4.0L engine, so I'm not as familiar with the 3.6L. Do the O2 sensors have the same part number? If so and you can turn your own wrench, you could try swapping the O2 sensors and see if the code follows the sensor from right side to left side. If so, it's the sensor. If not, then might indeed by the cat.

I had a misfire that I briefly drove on before having the coil (COP) replaced, and it took out my catalytic converter. I went with Eastern Cat after-market available on RockAuto for a lot cheaper than Mopar. It has held up fine. It is not a generic, but specific for our vehicles.


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