Im getting this P0171 code with the CEL light on only when its really cold outside, a couple of min's after driving. Never in warmer weather, above 30 degrees. I found the intake manifold throttle body hose split bad underneath and replaced it along with the air filter, disconnected the battery and after driving maybe 300 miles the light would come back on again in cold weather. Apparently theres something else causing the problem for the P0171 "bank 1 system to lean". Any thoughts on why the cel would only come on in cold weather after starting the car? This is a 2004 honda accord, auto, 162,000 miles. Also noted a rough idle sometimes at a stop light.

 

One possibility is an intake manifold leak that opens more when cold and closes up when warmed.

Suggest observing Long Term Fuel Trim (LTFT) w/ a obd2 scanner. If LTFT is high (perhaps 15-25% at idle) but decreases when rpm is raised to 2500, an intake air leak somewhere is likely. Air leak can be many places and leak detection w/ a smoke machine may be necessary to locate a leak.

P0171 can also be caused by a partially blocked fuel injector. This happened to me on a Toyota Celica. It is necessary to perform a fuel injector balance test to determine if there is a partially blocked injector. See youtube video by scannerdanner. This requires a special tool and fuel pressure test rig.

good luck