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Bryan, Technician
Category: Audi
Satisfied Customers: 410
Experience:  Four years experience diagnosing and repairing Audi vehicles
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Audi A6 Quattro: Hi There, I have an Audi A6 4.2 quattro.

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Hi There,

I have an Audi A6 4.2 quattro. The check engine light has been on for quite a while now. A mechanic looked at it and said that the Air Flow is low. I want know what could cause this. Would a clogged catalytic converter be the issue?


My name isXXXXX will be glad to assist you today in answering your questions.

I am guessing the mechanic said the air flow sensor was bad, this will cause a catalytic converter to go bad, a bad catalytic converter will cause the light to come one, just as a bad mass air flow sensor will. But normally a catalytic converter goes bad because the mass air flow sensor is not reading correctly. On your engine the catalytic converters were very good they normally did not need replacing unitl 200k miles. But the mass air flow was only good for about 6 years at best.

A clogged catalytic converter will choke the engine out, when hit the gas it would bog down or almost stall, because there is no where for the air to go. Also clogged catalytic converter will destroy your gas mileage, and the catalytic converter themselves turn cherry red.

Best regards,


Customer: replied 4 years ago.
Would this have anything to do with the O2 sensor? I just want to know what I should be looking at in order to get the engine ready to pass an e-test.

Engine management works like this

Mass air flow reads all the air coming into the engine, the engine adds fuel and the o2 measure all the air leaving the engine now the catalytic converter are supposed to take the hydrocarbons and add oxygen to them and turn it into water. So the o2 sensors measure oxygen left in the air leaving the engine. When the engine is running lean its burns hot and has excess air, when the engine runs rich the air has left over fuel in it and needs oxygen that is stored in the catalytic converter added to it.

O2 sensors last about 100k miles and should be replaced to extend the life of the catalytic converters.

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