Ok,check the data parameter for the PTO. If it shows that the PTO is active ,the computer will trim the engine rpm to a max lower rpm.
Here is a GM bulliten on PTO switch
PIP3187A: Engine Has Lower Power Check PTO Switch - keywords 6.6 6.6L 8.1 8.1L allison automatic diesel fuel L18 lack LB7 LBZ LLY LMM MAF no pressure RPM sluggishness sponginess speed - (Jun 9, 2009)
|Subject:||Engine Has Low Power, Check PTO Switch|
|Models:||2001-2007 Chevrolet Silverado Classic|
|2001-2007 GMC Sierra Classic|
|2003-2009 Chevrolet Kodiak|
|2007-2009 Chevrolet Silverado|
|Equipped with the 6.6 diesel or 8.1 gasoline engines|
This PI was superseded to update model years. Please discard PIP3187.
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
A customer may have a concern of low power. The dealer may notice that the engine will not go over 2000-2300 RPM.
A low power concern can be induced if the Power Take Off (PTO) switch is on.
If a low power concern has been duplicated, complete the current SI diagnostics for any trouble code or symptom found. During diagnosis be sure to check the PTO switch. When the PTO is enabled, it may limit the engine speed between 2,000 and 2,300 RPM (RPM that is reached may be dependent on actual PTO settings).
If the PTO switch is on, lower than expected readings on the FRP Regulator Fuel Flow Command, Turbocharger Boost Pressure, and Mass Air Flow readings may be noticed.
It may also cause changes in transmission operation such as long or delayed upshifts.
Evaluate low power after PTO has been disabled.