I've just done an up-pipe replacement on my 2005 blobeye WRX, swapping the standard one for a decat one. I've installed the 2k2 resistor to replace the EGT sensor.
As well as the up-pipe (the only modified part), I replaced the turbo inlet pipe (old one had a split in turbo end), a few of the smaller vacuum hoses around the purge valve that had turned to concrete, and the gaskets for the inlet manifold, intercooler and so on. Also replaced the gaskets from the exhaust manifolds all the way through to the downpipe. Basically gave everything a dismantle and clean (power steering fluid leaks, etc).

On restarting the car I now get a P0171 warning. The idle is a bit up and down too, but have just gone for a bit of a drive and the power seems fine (didn't have the chance to really let rip). I tried spraying a bit of brake cleaner around some of the hoses around the inlets to see if that picked up any vacuum leaks, but with the idle so variable it was difficult to tell if if make any difference.

Any suggestions?

 

You've removed the cat and increased airflow, standard ecu/boost controller doesn't know what's going on so afr is off hence the p0171, will likely need a map. A leak can cause it but I'd say its the removal of the cat

 

The error came up pretty much straight away on idle before the engine was really revved or driven. Would you really expect a different up-pipe to have such a drastic effect on idle?

 

Thats exactly when I would expect it mate.

 

I've just noticed my spelling of up-pipe in the title!

Anyway, apart from my inability to type, I have now discovered my inability to reinstall hoses properly. After trying out my homemade smoke machine to detect any inlet leaks it revealed this one. Not really the most subtle of leaks! So this is a big reason why I was getting the error message and rough idle. Annoyingly I can't test the engine yet as one of the pipes on the purge valve snapped off, so I'll just have more air leaking there.