I have a 2001 ML320 that is showing a PO422 code on the OBD2 scanner. That says Main Catalyst efficiency below threshold (bank 1). It sounds like there is a minor exhaust leak when its warming up, does this code mean I have a bad CAT or could it be a cracked weld in the system. Also is bank 1 the drivers side or passenger side of the car. Thanks for any info.

Bank one is the passenger side.

The W163 is well known for cracks in the catalytic converter, so that's possibly what's going on. Have it checked, as you know carbon monoxide is poison and it could leak into the passenger compartment.

MB cats from the late 90s to the mid 2000s are also known for failing; rattles, meltdowns, etc. That can obviously create problems, too.

Have you checked with dealer to see if you ML falls into ML catalyst recall?

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Ok, I share your pain.
I had the same problem, it was a cracked weld on the passenger side cat. I saw several forums where they removed them and had them welded. No one I could find would touch it, also the guts were rattling around inside. So after wrapping the weld in high temp fiberglass tape to get me through till I could decide what to do I bought an aftermarket setup that dropped in perfectly! He's on Ebay, muffler express is his name (mufexpress)

Its a day job, $350.00 (parts) or so, be ready to replace the rear clamp where it goes into the muffler. I cut the old assembly out with an angle grinder. He guarantees the check engine light will stay off, mine has for 9 months and I never changed the 02 sensor. I got $85.00 back at the junk yard for the old cats.

Mine was NOT under the warranty, it stops at 10 years, 80K I have 120K, Benz was no help.

Good luck, its not as bad of a job as you would expect.

-John

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John
2001 ML 320
1988 560 SL
2009 Toyota Tundra TRD
1992 400 E (sold '07) (bought back, '13)
2008 ML 350