View Full Version : 2000 a4 1.8t atw 11mpg!! p0422 code

02-20-2015, 03:14 AM
Roughly 2 weeks ago I noticed my car's fuel mileage had dropped very drastically. I thought I was leaking fuel at one point, but I don't smell anything and my driveway is always dry. I decided to hook up my vag-com tonight (CEL is always on due to SAI delete) and found p0422 on there. My display doesn't always work, but I was able to get a glimpse at it tonight and saw it was recording 11mpg @ 30mph consistently... I've been reading all sorts of posts on the p0422 code and everybody has a different opinion on what it definitely is lol. I only have the vag-com freeware version so my options on diagnostics are pretty limited to checking engine codes only. Some background on the car: 193k (runs like new!), flex pipe has leaked for the past 15k miles, new plugs/wire in the past 4k miles, new CCT, SAI delete, among other un-related maintenance. Also, the car has a slightly jumpy idle, jumps up and down 2-300 rmps while idling/stopped which gets worse when I have the lights, heated seats, fog lights ect. running. I have not been able to pin-point this issue which I'm sure it has a vacuum leak that has been eluding me for a while now.

I'm getting ready to order some parts for my 05' a4, and if I need to order a new cat, o2's, flex pipe or w/e for this car I'd like to do it all in one go. So if you guys have any insight as to what could be the result of a huge drop in mpg's along with this code the help would be very much appreciated! Thanks in advanced.

02-20-2015, 03:36 AM
Exhaust leaks eat gas. Winter also decreases mileage. Sounds like your cat and flex pipe require replacement.

02-20-2015, 05:45 AM
How do exhaust leaks eat gas exactly?

02-20-2015, 08:17 AM
yeah without looking at the car it appears you need a new cat with that mileage it maybe clogged. if you are looking to do a cheap fix you may look into gutting the cat and running o2 sensor spacers. while you're at it you should fix the flex pipe

02-20-2015, 08:24 AM
I think the exhaust leak gives the O2 sensor a lean reading, then increases the fuel ratio. Just think out loud.

02-20-2015, 09:35 AM
Wouldn't the leak have to be before the cat so it would there off the first O2 sensor?

02-20-2015, 07:50 PM
I personally, do not think the exhaust leak would decrease my gas mileage down to 11mpg. The exhaust has been leaking for quite some time, and I have always maintained 28-30mpg combined. The poor fuel mileage in my opinion is probably related to the p0422 code as they both appeared at the same time. The car runs about the same as it always has, I have not noticed any decrease in performance. The ATW has got to be among one of the slowest cars I've ever driven(lol).

I am going to replace the upstream o2 sensor, gut the cat, and replace the whole flex pipe with an oem replacement. The exhaust in the area surrounding the flex pipe is very rusty and my welder was blowing holes through the pipe and it was just a disaster. I have found it's world's easier just replacing the whole unit, rather than trying to put in a universal flex pipe.

- The majority of the exhaust leak is just past flex pipe, where the factory exhaust clamp is located. The only things I can think that would affect fuel mileage would be the upstream o2. A clogged cat, as far as I know cause poor engine performance. However, I've never encountered a clogged cat in any vehicle I've ever owned. Thoughts?

02-20-2015, 09:11 PM
I decided to remove the maf, and give a look at it. The metal surfaces inside the sensor have a nice coat of grime over them. Just finished cleaning it, not sure if this can cause the issues I'm having but I thought I'd give it a go.

02-21-2015, 07:25 AM
Wait, how do you know it's 11mpg at 30mph? What gear are you using? Where are you driving that this is even documentable?

You aren't reading the cluster gauge are you? Bc that would be silly..

02-21-2015, 09:57 AM
Of course I'm reading the cluster, how could I possibly know otherwise? I forgot to mention this car is an automatic. So, whatever gear it sits in @ 30mph is the gear I was in!

Without a fluctuations in the gas pedal at a consistent 30mph, the display reads 11mpg. The highest I have seen it jump was 18mpg while going down a hill.

I understand that it's not accurate. However, when I'm only getting 175miles on a full tank of gas.....prior to this, I could get 380 miles combined highway/city driving there is an issue and the display is not that far off. I currently own x3 1.8t audis as well as a 2.8 a6. None of which, get this poor of fuel mileage.

BUT..reinstalling the freshly cleaned MAF has significantly improved the cars idle. As far as I can tell the erratic idle is gone. However, I woke up to two flat rear tires, so the car is getting 2 new tires installed and I have not been able to drive it yet. (Bummed, we had a snow storm last night and my audis want to play in the deep snow :(.

I noticed the fuel gauge bouncing up and down from 1/4 to 1/2 tank while idling which is strange. The fuel pump/filter were replaced at the local audi dealership this past summer, they do not even have 2k miles on them yet.

02-22-2015, 09:31 AM
Of course I'm reading the cluster, how could I possibly know otherwise?
Well if you want to find out your average gas milage will out the computer you could just fill you tank then reset your trip meter and divide the miles driven by the gallons of gas used. also p0422 is a code that has to do with the efficiency of the cat so its probably has something to do with that. have you looked at the O2 sensor and or wires. on my car the wire fell out of the clip and fell on the exhaust burn right throw part of the wires. just an idea

02-22-2015, 09:41 AM
Of course I'm reading the cluster, how could I possibly know otherwise?

You average it out with your gas receipts. The cluster isn't accurate.

Know how many gallons your car takes, fill it up, reset distance meter, see how many miles you fill up on next refill, and you can play with the numbers to get a rough "real world" estimate of gas consumption.


13 gallon fill up
You clocked 364 miles till next refill
Next refill you fill up 14.3 gallons

364/14.3=25.45 mpg