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Discussion Starter · #1 ·
Hi all,
I've been following the forum for a while now, since I just got a 2004 Passat 1.8L GLS sedan recently (at 113k).
After 6 month of moderate driving, I got CEL, which was diagnosed as oxygen sensor problem (pre-cat).

So I got it replaced (as well as the thermostat, which seems to be an irrelevant issue). And now I have absolutely terrible fuel economy. I won't be able to give you numbers because I just realized this and wanted to ask if this is an expected/temporary issue shortly after the o2 change. I've driven around in the city less than 40 miles, and the tank went from half full to nearly empty.

I was expecting way better fuel economy and now I wonder if I should get back to the guy who repaired it. Did anyone have a similar experience after o2 sensor change before?
 

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Hi all,
I got it replaced (as well as the thermostat, which seems to be an irrelevant issue)
A thermostat that is stuck open would cause bad fuel economy. Check to see if your engine temperature rises to the normal range as quickly as usual and stays there. If not, the thermostat is bad.
 

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Half a tank in 40 miles? Sounds like the new sensor might be horribly defective (or the wrong one.)
 

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Discussion Starter · #4 ·
@tallngangly: that was the problem with the thermostat when I took the car to diagnostic. Told the mechanic exactly that and he replaced it. Do you think there could be something that went wrong during the change?

@sirwired: Yeah that goes without saying. The question is this: The sensor seems to be part of an electronic system, and I was wondering if the performance of the car prior to change can have an effect on the performance. I'm wondering if the issue would be resolved in time on its own, or do I have to reset something...Or if it definitely requires some more mechanical attention.
 

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Discussion Starter · #5 ·
Also, is there a rule of thumb to know if the temperature rises at a normal speed? What's the nominal range for example?
 

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In temperatures normal to summer in USA (have no idea where in the US you are), your car should come up to 190 which is right on the middle, in a mile or less.
 

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what part of RI are you in? I can run a VAG-COM scan for you.
 
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