My car keeps throwing the P0171 code. It is happening when I am sitting at idle for longer periods, usually with air on (drive thru with long line). Car is driving fine but can get a little erratic at idle at times too (shudders a couple of times at stoplights.) Where should I start looking first? Car has 75k miles and just changed spark plugs. Thanks!

 

Clean the MAF, clean the throttle body. Carry out a KAM reset, go for a drive, get the engine hot and find someone with a scan tool that can monitor live data & check the Air fuel sensor & O2 sensor.