SES woes - Could use advice

[Stillwater Cowboy]

I bought a 2000 GMC Sierra SLE Z71 for my son's first truck. It has the 5.3L and has 237,000 miles on it. It is pretty darned clean and has been well taken care of. The Service Engine Soon light was on when we bought it, and had the codes checked. It showed Bank 2, Sensor 2 so we put in a new O2 sensor. This didn't fix the light so we had the GM dealer check the codes. This revealed a catalytic converter low efficiency, so we had a new cat placed on the right side of the truck. The light was finally off, and stayed off for about four days.

When it came back on, I took it back to the muffler shop to have the codes read, which revealed P0171 System Too Lean Bank 1 and P0174 System Too Lean System Too Lean Bank 2. The muffler mechanic reset the codes and the light has stayed off for a while. If it returns, what is my best next step to diagnose?

We checked the air filter. it's clean and the box was bolted down properly and the air filter was properly fitted in there. Should I clean the MAF next? I've read that I should check for vacuum line leaks on the intake, and then the fuel pump. The previous owner replaced the fuel pump a while back and it isn't showing any symptoms of fuel pump failure. I just want to be smart and spend my dollars wisely.

Any help would be much appreciated.

Thanks!

Stillwater Cowboy

 

[98trck]

have you run some quality fuel injector cleaner on several tanks of fuel,
may have to have the injectors pressure cleaned at the dealer

 

[GMCGang]

I agree run some good fuel injector cleaners on multiple tanks of fuel.

 

[Bill325]

Fuel injector cleaner

I've had very good luck using Chevron Techron fuel system cleaner, and Chevron Techron Fuel Injector Cleaner.