04 Pontiac Vibe bad gas mileage without a code and then the p0420

Make sure the air tube between the throttle body and the air filter box is not loose or popped off .

take out the spark plugs and check them also check the pistons for carbon build up, check the vacuum lines for leaks and being clog or choked espaly the top pipes as they can get clogged up on the breather line. also check the sump breather pipe as this can clog up over time. remove the belt auxury and test the movement on the power steering (if fitted) should move easy and do the same with anything else on there like alternator etc. anything that is stiff to move this can add more fuel use. even an oil change can help with mpg.

is this calculated mileage or what the estimated mileage is showing on the dash. Have you checked to see what you get on just the freeway?

What are your driving habits like?

How is your tire pressure?

Do you make a lot of short trips?

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I believe it is something they are putting in the gas. I have 2 cars that are getting worse mileage now than in the summer. 1 is a Prius the other is a Pontiac Vibe. The Prius used to go 450 miles on a full tank now i'm lucky if i get 300. I also asked a co worker and he said the same thing, less mileage. I wouldn't get nuts trying to find something mechanically wrong with your car.

why were the sensors replaced? what symptoms do you have, does it have a long cranking time before it runs?

Cause you're driving a bus. lol. Mileage will vary depending on what fuel you use, how you drive, and where you're located. typically what i've seen on trucks like yours, i'm asuming it's a 6.0 liter, is anywhere from 12-19 mpg. you should see around 15mpg for the most part. if you do a lot of in town driving, expect a lot less. @9-12. if it's less than that, i'd start lokking in to it. try running a K&N air filter and use synthetic engine oil. I've seen noticeable improvements with power and mpg's with that. Also, ensure that your tires are properly inflated as that can make a difference in mileage. use the recomended fuel octane and change fuel filters.

Hi,

For P0420 and P0430 ,This means there is a issue with the catalytic converter and O2 sensor being no longer efficient. You are going to have to get them tested. Until then you will start seeing worse gas mileage and the car is going to not have as much power the longer you wait the worse it will be. You can get this done at any VW dealer. They have to hook up a scan tool and monitor the system. There are a few outcomes and its very costly to just start replacing parts. Its either bad wiring, O2 sensor is bad, Cat is bad, also could be a bad ECM the shop will be able to pinpoint the issue and know the exact item costing you less in the long run. Once you find out dont be obligated to spend full price there you can always have a local shop do it for less or do it yourself and order the parts online

For P0441, You won't notice a decrease in MPG, this system is the charcoal canister system that absorbes fuel vapor from the fuel tank so when you open the cap you don't vent fuel vapors into the atmosphere. The code is saying for some reason when the car triggers the valve that opens allowing these vapors to be drawn into the intake, it isn't seeing an increase in airflow. Most likely the purge valve has failed. It's a little valve with an electrical connector and two rubber hoses running to it on top of the air filter box. You can disconnect the hoses and check to see if there's charcoal contamination in the valve by tapping it on white paper.

Also you can check your vacuum lines to the valves on the back of the cylinder heads (they have the large plastic hoses running to them).

It is safe to turn off the code but the light will return. VWs don't throw phantom codes.

Let me know,if needed further assistance.

Hope i helped you.

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I had the problem with my 04 silverado odometer malfunction. There was a recall for this model and year. Took it to the dealer had it fixed for free, you need a new odometer replaced but it only covers warany for 7yrs or 75K miles then its prorated.

replace air filter, slowly accelorate, keep 60mph or under, lower rpm,s, don,t use gas or brakes as much(coast to stop) check and clean fuel injection system (use cleaner) 

A really dirty air filter will knock down your gas mileage greatly....yes. Also keep in mind that you don't get as good of mileage around town, as you do on the freeway.....35 mpg isn't too shabby....oh and please give me a rating....thank you