I recently took my 2000 E430 4matic in to the dealer to address the CEL. He replaced the left rear o2 sensor and this seemingly solved the problem. Now 500 miles later the cel has come on again. I am wondering if the MAF could have thrown the o2 sensor fault in the first place and maybe that should have been replaced instead of the sensor. I am planning to call the dealer tomorrow but was looking for info before I do. Thanks in advance.

IF you post the codes...... we can help with definition...

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

The new code is P0173, and the original code was P0161. Now I'm really starting to think that the maf caused the prior problem.

check

Mass air sensor won't cause rear O2 heater code!

P0 173 will mean defective MAS 99% of the time!

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Thanks MBDOC ... that's what I needed to know.