Komatsu PC 400-6

Just purchased this hoe and it is having trouble lighting itself in the boom down operation. The main control valve was changed out by komatsu dealer along with the main pump and complete new engine approxamatley 2500 hrs ago. What would you recommend be the steps to diagnose this problem? I believe there is a boom down control valve and I'm guessing the other thing to check would be the lift cylinders. Any help our thoughts would be appreciated. Thanks.

It is having a hard time lifting itself

The control valve is a new or reman?

On the right hand side beside your arm in the cab there are a number of switches. Switch #8 or the last one closest to the back should be Machine Push Up Switch. If it is switched to the low setting or is faulty you will have low thrust force on your boom. ( I would check that first)

Brand new control valve. I will check the switch. Do I just check input and output voltage on the switch? I'll have the machine at my shop by the middle of the week to test. Thanks for the suggestions

If the switch is shorted it will be constantly on the low setting and if you remove the switch, it should go to high. Now, if your switch checks out ok I would then make sure that the spool on the solenoid valve(V05) is not sticking/stuck.