VW passat mileage dropped down suddenly

My car mileage dropped down suddenly this week. It was always running at 10 km/litre and this entire week it ran at 6-7 km/litre. I have been putting premium gas, no change in driving habit. But what is the cause of sudden drop in mileage? Also i got engine light, and not going off. Many times before it appeared but i went off automatically within 1/2 days but this time engine light stuck on my dashboard from last one week.

Figured out the message is faulty catalytic convertor.
Can anyone help me with this

what year is your vehicle? and I am guessing the check engine light has a lot to do with it. can you post the engine codes?

That could be the result of much lower winter temperatures, or it could be the result of engine problems.

Until you take the car somewhere to have the stored Diagnostic Trouble Codes “read”, nobody from afar can accurately diagnose the problem. Clearly, there is a problem, but until you find out what codes are present, there are many possibilities as to the source of the problem.

I surmise that you live somewhere other than The US, so things could well be different in the nation where you reside, but in The US, most auto parts stores will “read” those codes for you, and give you a print-out. Give that a try and then, you can return to this thread, post the codes, and get more specific guidance.


It says something wrong with catalytic convertor. Its 2014 VW Passat

Do you think changing oxygen sensor might help?

Which one of the 53 codes does it say for the catalytic convertor? they all mean something different

1 Like

Agree with above. a bad O2 sensor can cause those symptoms, but other things can too. you do not want to keep throwing parts and money away to fix the vehicle. best to post codes to get better answers. even better to bring it to a reliable shop to get it diagnosed. just not a chain shop.


Yes i guess its o2 sensors. I will check with mechanic thanks everyone for replying.

P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1) (obd-codes.com)

If it’s only P0420 that is strange.

I would think that the upstream oxygen sensor failed, but for some reason did not trigger a fault code. I suspect it’s causing the engine to run too rich and reduce the fuel economy. O2 sensors measure how lean the engine is running, and when they fail they under report how lean it is, causing the engine to run too rich.

Now that the down stream O2 sensor has sensed a failed catalytic converter, it could be that the catalytic converter is damaged, which is unfortunate. Hopefully it’s not damaged yet, so get the upstream O2 sensor(s) replaced right away before it does damage it and you waste any more money on fuel.

If you can’t replace it right away, then unplug the upstream O2 sensor before you drive it much more.

I just now realized that this is an old thread. Maybe someone else will get some use from it.