Not really. A MBC simply limits the amount of boost pressure the turbo can develop. Though if you spend a lot of time in boost (higher than 0psi) that would attribute to poor gas mileage... but this is independent of a MBC.

Generally, these things can cause poor gas mileage:
-clogged fuel filter
-dirty fuel injectors
-old/misgapped spark plugs
-old primary O2 sensor
-boost/vacuum leak
-dirty air filter
-dirty or malfunctioning MAF sensor
-clogged catalytic converter (not your issue obviously)
-maybe low compression?

 

Not having a cat should not cause the car to run rich (typically the car actually runs better, afaik people generally see horsepower gains), but it will cause an inefficiency code and you will obviously fail emissions if they apply in your state. They do make non-oem universal cats but they are usually at least about 300 dollars, OEM cats are probably around 1000. A the primary oxygen sensor (Note that it is NOT the sensor that is giving you the code) could cause you to run rich. Does your car start on the first try?

There about 4 or 5 reasons why the A/C compressor will turn off: an Engine Overheating condition, an outside temperature of less than 34 degrees F, an activated "kick-down" switch (you are at wide open full throttle, we can rule this out because it wont cause the snowflake to dissapear and will only last a few seconds) and a refrigerant pressure that is too high or too low. If your outside temperature display is above 34 then your issue is likely with the pressure.

To diagnose this you must first check the A/C pressure, they make cheap gauges to do this that connect to the low side of the A/C system. If the A/C pressure is too low or too high the system will turn off to prevent damage. The most common issue is a leak in the system (often times at the "condenser", the black radiator in front of the actual coolant radiator). The leak must be fixed and then the system has to be refilled (preferably "evacuated" and refilled). If the pressure is good there is likely a problem with the A/C
"trinary" switch. There is a single switch that monitors pressures, it has three positions hence "trinary", OK pressure, low pressure and high pressure.. If this switch is faulty it will prevent the A/C from running. You can verify this is the problem AFTER you rule out a leak in the system by jumpering pins on the harness that runs to this switch (only briefly because the purpose of this switch is to protect the system). I am almost 100 percent positive that the system needs to be evacuated before replacing this switch. This switch is located behind the passenger side headlight on the passenger side of the "condenser" and has a square connector.

EDIT: And that ABS code is usually a result of a bad ABS control module.. Do you have an ABS light on, and also does the ABS light come on when you start the car?

 

This is one of the threads that I was reading where someone said no cat = running rich
https://www.audiforums.com/forum/b5-...t-pipe-180463/

Not that I'm saying I don't believe you guys, it's just a bit confusing...

Anyway rich or not the fumes are awful, definitely not the smell of a normal old car with no cat. I'm planning to get a 300 bucks cat but I gotta find someone who is gonna install it since this mechanic doesn't want to. Actually if anyone has a suggestion for a good audi mechanic in the Buffalo area it would be immensely helpful!

About the causes of the low mileage, thanks for the list!

For the ABS I don't have a light on, and I'm pretty sure it shows up when starting the car. I gonna check when I get the car back.

 

I would look for an exhaust specialty shop to do the work. AFAIK there is no reason you would run rich, only appear to run rich from the increase in emissions, smell, soot.

 

If the boost gauge goes up to around 5 psi when the turbo is engaged, is it likely that there's a boost leak?

 

That or a fault in the engine is putting you in safety boost.

 

ok, calipers dont get "worn out" a piston is either seized in the bore or the slides are froze up. one of the 2. if your exhaust has a sulfur ish smell, you can bet ur bottom dollar the cat has been clogged up. ur idle air control valve needs replacing. boost psi control valve makes me think the wastegates stuck open. look for coolant leaks around the turbo and pray you havent blown a headgasket, check ur oil for residual coolant floating around, dipstick and oil fill cap. if not that then id imagine a radiator leak/heater core leak. do you smell coolant at anytime?
o2 sensor before cat monitors/adjust air fuel ratio. the post cat, monitors simply wether the catalyst is working properly or not.

 

Yeah the calipers were seized but that's fixed now.

Why you say that the idle control valve needs to be replaced?

I think you might be right about the wastgate being stuck open. How easy it is to reverse the boost manual control and bring it back to auto? I don't seem to be able to find a good step by step on this...

 

The car doesn't start right away, but I've gotta say that the battery is pretty low, if I don't turn on the car every other day it doesn't start at all (but its pretty cold these days and I've not been using the car much). Why did you ask?

About the ABS no the light doesn't come on when I start the car.

Thanks!

 

Does the brake light come on if you set the emergency brake? It sounds to me like someone has probably removed the ABS LED warning light from your instrument cluster.. preventing you from knowing if their is an ABS fault (a cheaper cover up than actually fixing the problem). It is likely that it would be on right now based on the code you gave, meaning it is also likely that you do not have brake proportioning (making it more likely for the back end of your car to kick out under panic braking) and anti lock brakes (making it very likely your brakes will lock under panic braking).

EDIT: To answer your other question, hard starting (with a good battery) could be a sign of a leaky fuel injector or a bad coolant temp sensor, both of which could give you bad gas mileage.