My truck is sending this code and says Lack of HO2S21 Switch Sensor indicates lean can anyone tell me what parts I need to change to fix this problem.

Thanks Art

 

Welcome to the forum

A P1151 means that the front O2 sensor on the driver's side is reading lean and the PCM can't add enough extra gas to correct for it.
Does the truck run well? Does it idle rough or misfire or anything like that?
A vacuum leak can cause the problem but usually you'll have rough running if that's the case.
A misfire in one of the cylinders on the driver's side can also cause it but usually you'll get a code for that too.
If it runs well it's probably safe to replace the O2 sensor.
The best way to diagnose it would to watch the live data from the sensors on a scan tool capable of doing that and see what shows up.

 

It does not run rough but sometimes when I come to a stop it Idles rough but runs fine under power maybe I should replace my fuel filter the truck has 61000 miles on it and I dont think the o2 sensors or fuel filter have ever been changed.