So the other day while driving i got a CEL. Drove to autozone got it tested and it turned out to be p0137 low circuit voltage on B1 S2. This is after the cat.

I took it to a trusted mechanic and he said it's most likely a bad o2 sensor and it'd be $225 to get it replaced with OEM.
I said ill take care of it later and he manually turned the light off.
10 miles down the rode it came back on, which was expected.
Little something i forgot to mention, i just got this car (1998 a4 2.8l) about 3 weeks ago and i put in regular fuel, which is fine from what i read online.
Anyway, later that evening after going to the mechanic, i put in premium just to see if there was a difference in how the car ran. couldn't tell a difference but after about 10 or so miles, the CEL light went out.

wtf happened? was it the fueL?

 

Fairly certain these cars are pretty testy when it comes to fuel. I have - and would advice others - to only put in premium. I've heard of others putting in regular and hearing a knocking and getting worse gas mileage. Never heard of rear o2 sensor issues. I DO know that if you put in 95 or 97 it can melt the pre-cat o2, but I dunno if that has an effect on the post-cat.
My advice would be to just leave it. Continue to put in premium and chalk it up to the gas, or maybe it was some odd fluke.
My guess is that it will come back and you'll have to have a mechanic replace it.

 

1.) Electronics can become "crippled" before they finally die. Your O2 sensor (if that's what's actually wrong) could be having that problem. That would cause intermittent functionality.

2.) Never put lower grade fuel in a car that requires a certain octane. The purpose of octane ratings relate directly to compression ratio. A good rule of thumb is if you take your compression ratio and move the decimal point over to the right by one digit, you'll get the octane rating that will bring you closest to a stoichiometric ratio, which would yield the most power and the greatest efficiency (efficiency directly effects power), so if you have a 9.5:1 compression ratio, you should be running 95 octane. 10.5:1 would require 105 octane, but car manufacturers know that some states only offer a max of 93 octane etc., so they retard the timing (aka: de-tune the engine) to accommodate that. When you run lower octane than what's required, it ignites before the piston is at TDC, which causes detonation, causing a loss of power, fuel economy and most importantly, creates the very real scenario in which you can cause one (or more) of your pistons to explode.

If your fuel door say "use 93 Octane" - use 93 octane (or as close as you can get). If you can't get what's required, you can get lead substitutes, or upper cylinder lubes (like Lucas UCL).

I hope that helps.

G/L