I just picked up an '03 MCS with 111,xxx miles (my 3rd Mini) about 2 weeks ago. It has CAI and Borla exhaust but besides that appears to be stock. About 2 days in it threw the SES light, code P0171 and P0131. I assumed it's an O2 Sensor and replaced it with the factory NTK unit. Also replaced the plugs, wires and coil. Everything seemed good and then SES code P0171 again. I've tried Octane Booster (already running 93) and no help.
I can't get the car to pass inspection without the light going off. I checked for exhaust leaks and there do not appear to be any. Also checked a lot of the intake hoses and still no dice. Anything I might be overlooking? The plugs all looked decent when I changed them, but the coil did have a lot of corrosion on the cylinder 3 point. I guess it could be a fuel delivery issue. With the light reset it still drives great and doesn't ping or anything even when driven hard. The idle turns rough when it throws the light again and then it's sluggish.
I'm running out of ideas and an hour from the closest dealer. There is a local shop that works on them some, but they are closed this week. Starting to regret this purchase (except when I drive it).

 

could be pcv valve. I'd be looking at vacuum lines too. Also my first oil change done by MINI gave me that code, turned out the oil cap was loose.

 

Good article about this.

https://repairpal.com/OBD-II-Code-P0171-and-P0174

 

Make sure you check all your vacuum lines and the PCV valve as stated above. I had this issue one time and it was caused by a broken vacuum line close to where I had installed my oil catch can.
Do you have a Boost gauge installed? Please check all the lines routed to the gauge. There is a vacuum hose right under the passenger side intercooler horn. Check that that is in place too. Hope that helps!

 

Thanks guys! I see a day of working on the car ahead of me tomorrow. The air intake hose from the air filter box to the throttle body is a tad loose at the TB side (can make it move some if pulling at it). I also noticed the PCV is not a stock one and doesn't even have a flange to bolt it to the valve cover. I think I'm going to start with replacing that valve and the intake hose. Going to order an oil cap seal right away too (for $5 why not?). I can't hear any leaks anywhere, but with the CAI it's hard to hear anything else.

It does have a bit of a rough idle even with the codes cleared (like a V8 with a big cam). I just wish I knew what all was done to this car. I'm hoping to get it hooked-up to a computer and hopefully have it tuned. That might fix everything. Does anyone know any places near Eastern PA that do Mini tuning?

 

Ok, so after more looking around and some cleaning, here's my progress...

-Found a loose clamp from the air intake to the throttle body. Fixed/cleaned all pieces.
-Removed and cleaned PCV and corresponding lines.
-Reset all codes.
-Took to the only local shop that has software to diagnose a Mini. He told me it is running too rough, but he can't find a real reason. He fears the timing belt is slipping. Doesn't it have a timing chain? I assume they usually just snap and fail, not slip.
-SES light has not come back on (over 50 miles and 9 restarts)
-Stopped at my garage to try and get emission sticker and it shows that code P0171 is pending and they can't pass it yet.

Looks like my light will be back on any day now, ugh. He suggested a seafoam treatment hoping that will clean out everything and maybe fix all issues. Guess that's my next step unless anyone else has a better/other suggestion.