I assume you have a 1.8T if you were under the coil recall? You'll get better answers if you specify engine and year when posting.

P0421 is more common, but both can indicate a bad rear oxygen sensor, a bad catalyst, or a running condition that made the CAT appear bad. Assuming your car is running smoothly now (after the coils) I'd start with a new rear O2 sensor. If not, the CAT may need to be replaced. Now, catalytic converters are theoretically lifetime items, but they can be made to fail prematurely. If your engine isn't running properly - possibly even from bad coils! - it can ruin the CAT. So can overfilling the crankcase, running for an extended time with a stuck thermostat, or having an internal oil leak that gets oil into the CAT. I've suffered from all of these myself.

Good luck!

 

Scott, how does a stuck thermostat cause cat to fail? I just bought my B5.5 1.8T (AWM) 90k miles about two months ago, WITH A STUCK THERMOSTAT. Replaced timing belt, thermostat, water pump etc, car ran perfectly then the cat failed (misfires on all 4 cylinders, severe back pressure and loss of power) after two weeks of driving it. I have replaced the cat (034 motorsports hi flow with rear O2 spacer) and it failed Colorado emissions due to high NOx levels. Meantime I got a P0420 code just once, cleared it and it hasn't returned. I took it to the dealer yesterday for some recall items (fuel pump, fuel lines, exhaust heat shield and four new coil packs). Hoping the new coil packs will cure the NOx issue but am doubtful.

 

Thank you for your quick reply. I did forget to state what engine type, oops. I took it to the VW dealer to remove the wheel locks since it wasn't in the car and the dealer informed me about the recall on the coils. I will start with a rear O2 sensor and go from there. Thank you again for your help I appreciate it.


Respectfully,
Fred

 

i wouldn't just throw an O2 sensor at it, I'd first have the fault code cleared, and try driving it for a bit, take it on the highway and rip it pretty hard, you might be able to clear it out a bit, it's cheaper than an O2 Sensor, and if the fault comes back for poor catalyst efficiency, then I would say you need a cat

good luck

 

You 02 sensor is fine, its doing its job in detecting a problem, you need a new catalytic converter.

I had the same code and drove with it for 6 months no problem, just annoying. Not a priority issue.

 

IF your 02 sensor is fine, its doing its job and detecting a problem, and you need a new catalytic converter.

^^^ made some changes.

More than one thing can cause this code to pop. A failing CAT is one of them. A failing #2 O2 sensor is another. Replacing an O2 sensor is less expensive than replacing a CAT. I agree that you should first try clearing the code and see if it comes back.

In some areas this code will keep you from passing emissions testing.

 

atp160, do you have a Check Engine Light lit, or just the code showing up on the dealer scan? A CEL from this will turn itself off pretty quickly if the sensor reading comes back within bounds (the code will still be stored). RE: Tech 71's advice, my old 2000 had a marginal CAT and the P0421 would come and go depending on how I drove. No need to re-set.

 

I had the exact same problem 2 months ago. My 2.8 V6 never had any problems before.
The dealership wanted my left hand and leg to repair it. Long story short they reset the light and I switched buying gas from Arco to Chevron. Light has not come on since. Changing where I get my gas was my idea not the dealerships. Hope this helps.

 

hone out the cat. then see how it runs, then when you have money to change it change out the cat.

 

P0420

Ok, first the code means that the rear oxygen sensor is getting a faster reaction that it shouls be. That means its is working. If the car is almost ten years old more likely its going to need a catalytic converter. If you doubt it buy a VAGCOM and then perform basic settings for the front and rear O2 sensors. And when the rear will not set then you will know that the Catayltic converter is faulty. Otherwise, go with what the fault code is for... the catalytic converter. It maybe covered under warrenty depending on if you have the 10year emission warrenty on some of the older cars.

 

A catalytic converter is supposed to operate under a fairly narrow range of conditions. Right temperature, right mix of gasses out the pipe, not too much uncombusted hydrocarbon. When the thermostat sticks open, the engine doesn't reach operating temperature promptly, or in winter, at all.

The engine sensors see a cold engine, and richens the mixture until it warms up - which never happens. So, too much hydrocarbon goes into a cold CAT, isn't catalyzed, and it clogs up. Worst case, if you get too much gas back there (as when the engine misfires), it combusts in the cat and ruins it in minutes instead of months. In the case of my car, running through a winter with a stuck thermostat cost me a CAT, plus more clogs in the PCV system, higher fuel consumption, etc.

In your case... if your NOx is high, you have a lot of possibilities. It may be a bad or incorrect CAT; not all setups work with aftermarket CATs. The fact that you have to trick the rear sensor means you have a real problem. More likely, though, are a lean fuel condition (from vacuum leaks, clogged PCV system, bad MAF, etc) misfires, exhaust leaks, or even a bad forward O2 sensor.

Unless you're throwing P0300 series codes from misfires, coils or spark plugs won't fix it.

 

Thanks Scott, makes perfect sense. Previous owner didn't drive it much the past year or so but knew there was a stuck thermostat, bought one with outlet, left it the trunk for me, I don't think he had the ability to do it himself.

I figured the new coil packs wouldn't solve anything (re: high NOx) and have already targeted the PCV system as the first possible culprit, seems to be a weak point. Could be causing the slight leak at the cam tensioner seal too.

Thanks for all of your help.