I'm working on a 2018 ram 2500 this weekend, truck is lifted with 37's but drivetrain is all stock except for an MBRP DPF back exhaust with 57k miles.  Owner recently noticed a severe lack of power and it makes very little boost.  I took it for a quick drive and with the pedal stuck on the floor it was like a pleasant sunday drive.  The truck will still get up to speed and maintain it but takes forever to get there.  I didn't notice any smoke from the tail pipe.  It has an engine light on but I went to look at the shocks, so I didn't bring my scan tool.  He's bringing it over sunday so I can take a better look and see what the light is about.  I'm wondering if these symptoms point to anything typical of these trucks to better prepare me for when it gets here.  

Bit hard to tell with no code. Emissions are suspect, my first guess is the EGR system.  

that or vanes on the turbo stuck open.

I never even got to check.  He cleaned the filter and an intake sensor and his buddy cleared the codes before he brought it over.  It had more power but he says it still isn't as strong as it was.  He had bought a new K&N filter to put in it but I advised him against it.  His job has him off road 90% of the time and I see how much dust passes through those things with city driving.  

I agree with the above, but will add that I would be suspect of the MAF sensor, considering he had a K&N in there. Most OBD tools will allow monitoring the Mass air flow.  It's possible the MAF is reading low, not allowing the ECM to fuel, spool and go.

How old are the fuel filters?

 

Does he know there are 2 to replace?