lcc014

CEL came on yesterday and the fault code was P0171. I just had the knock sensor replaced due to another code.

Any idea to tackle this problem ?

Thanks,

Ching-Ho Cheng

moonphase

According to the Haynes manual, P0171 system too lean, and the causes could be any of the following:
loose air intake hose or duct
incorrect fuel line pressure
obstruction in injector
heated oxygen sensor malfunction
MAP or MAF sensor malfunction
ECT sensor malfunction

tomf

Assuming you assembled your engine correctly after replacing the knock sensor, If I were a betting man I'd say it was the Bank 1 Sensor 1 O2 sensor.

But first thing I would do would be to clean the MAF. It's easy enough to do.

Second look for any obvious vacuum leaks. Did you forget to connect a hose or other connection after you replaced your knock sensor?

third replace the Bank 1 Sensor 1 O2 sensor.

O2 sensors are notorious for failing on the 1MZ-FE engine, usually sometime after reaching 100K miles.

My personal experience has been in 3 1MZ-FE engines that P0171 is a O2 sensor failure. Other people will disagree and say it's more likely the MAF.

mdbrown

Check for an air leak around the intake boot.