2008 f350 6.4 liter wrench light and reduced engine power on no codes

What codes do u get before u clear the computer?

if they didn't read the fault codes before starting work, they are repairing by replacement and that is not good
if a sensor is faulty and it could be a cam shaft sensor as well as a crankshaft sensor , normally it misfires or stops altogether
the ECM is in limp mode and that reduced the engine power and if there was a sensor problem it would not just reduce power and put the light on but would show up in a define manner
engine lights also come on it there is a transmission problem and that will also initiate the limp mode
have the fault codes read to determine the faulty part/ sensor

Check engine light on , check DTC'S - diagnostic trouble codes ! Check Engine Light On If not familiar with this take it to a ASE certified repair shop or dealer . 3rd limp mode ? Do you have a message in the DIC - driver information center , reduced power mode ?
ENGINE POWER IS REDUCED
Engine Control Module Description for the 4.8L, 5.3L, 6.0L and 6.2L engine

Jerry, Drive to a good shop. So many things can cause reduced power light unfortunately. The first step would be to get the truck hooked up to a scanner/computer and have the computer scanned for trouble codes. One thing that you could try is to drain the water from the water separator that is mounted on the frame under drivers front door. If the PCM picks up on water, power is reduced until water is removed/drained. It could be an exhaust sensor that is reading faulty. Unfortunately from here its impossible to tell you exactly what the fault is because there are just so many engine sensor inputs that could cause your problem. Check the attached links, forums,instruction and guides, Good luck
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Reduced Engine Power Message Displayed on GM Vehicles

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Dec 10, 2012 - Tech Tip: Reduced Engine Power Message Displayed on GM Vehicles ... Model year 2008-'10 vehicles must have the engine control module (ECM) reprogrammed ... Do not drop the electronic throttle control components.

Have the codes read. I haven't worked on any thing to date that reduce power because of the coolant system or the oil. I have done some that will kill the engine because of both, but not reduced power. This could be from an engine hard part, sensor or wiring. Most times it's caused from a transmission problem.

My code on my van P0641 does anybody know what that mean

There is a GM service bulletin on this particular item. basically covers both codes your getting.
#07-06-04-019A: Intermittent MIL/SES, DTC P2138 with Reduced Engine Power (Repair Instrument Panel (IP) to Body Harness Connector)

Models: 2005-2008 GM Passenger Cars and Light Duty Trucks (Including Saturn)