I've got an ML350 in that comes in with the low catalytic converter efficiency code on both sides. After a 15 minute ride I put it on the lift and measured temperatures on the inlet and outlet side of each cat using a handheld IR gun measure at approximately the same point on both sides. Temperatures are as follows:

Passenger side:

1st cat (O2 sensors are on each side of it)
in 284 dC, out 209 dC,

2nd cat
in 201, out 187

Drivers side

in 253, out 284
in 200, out 180

Any way to tell by these temperatures if either of the cats are bad and if so which one. Car runs/drives good but I suspect it is using oil which can lead to premature cat failure from what I have read.

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Jim