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Discussion Starter #1 (Edited)
**I preface this by saying that I always run my tanks of gas to empty before filling up again, and always put the same amount of fuel in there by volume, not by $ amount**


So, once after filling up with gas and slightly overfilling the filler the tube, two miles down the road and the MIL came on. Checked with forscan, P0420 - catalyst below efficiency.


cleared the malfunction, drove the car over 900 miles, 3 tanks of gas, no light, all readiness monitors completed, emissions ready


another week passes, another 3 tanks of gas or so [work travel], no light.
then I overfilled the filling tube when filling with gas again one day. two miles down the road, MIL comes on. P0420.


go through the same procedure as before, but this time I replaced the Evap Purge Valve in the engine bay, as my part was still the "B" part, whereas the new one is a tube and solenoid only without the air bridge and ending in "R" for the latest revision of the part.


no more light. CRAZIEST THING is that my car drives like an entirely different car now. somehow that valve must have been malfunctioning the whole time I've owned this car (second owner, 23k miles to present) because I can tell that part was changed.

So this made me investigate further to make sure my gas tank was not deformed or collapsed per the TSB on the subject, which thankfully it is not.


TL:DR = if you have intermittent P0420 and you haven't replaced the purge valve with the updated part, do so before you replace the pre-cat or a shop does. part cost me $25 with a dealer discount, and about 30 minutes of my time because the connectors are a HUGE PIA to disengage, and to reinstall by the throttle body
 

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Same problem with mine about a month ago. Replaced the canister purge valve and it's been running perfect since.

Sent from my SM-G950U using Tapatalk
 

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Don't overfill is the key also. Pressure builds in your tank and need room to expand of it forces fuel into the purge canister and saturates it and will need to be replaced. This is a common issue on a lot of makes and models.
 

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I only ask cause i recently got a p0420 myself, and all current data looks good. Upstream, down stream, fuel trims etc. Cat temp in freeze frame is about 1400°
 

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Discussion Starter #7
How long have you driven now, with no return of dtc?
light came back on after 5,196 miles after being reset after new purge valve installation. that was two weeks ago, now. light is still off and all readiness monitors are complete.

no big deal. just means the cat is slowly failing. just glad to know it's not the gas tank collapsing on itself which i've seen now on several mk3's at work.

but, i still suggest to replace the purge valve before going any further.
 

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Discussion Starter #8
UPDATE:

I finally broke down and bought a new downstream o2 sensor, and a o2 sensor bung extender. I removed the old downstream o2 sensor, put in the extender, then installed the new o2 sensor.

P0420 completely gone and has not come back on for over 20k miles, and no impact negatively to performance or gas mileage.

so, don't replace the manifold catalytic converter unless it is diagnosed as breaking apart and clogging your exhaust line, then you should remove and replace. in my case this was unnecessary. o2 bung extender works perfectly.
 

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