Search this website's forums for "pedal dance" for directions on clearing the code using the accelerator pedal and a stop watch.

In step 1 over heat [code e-32], the engine speed is "derated" and a limp home max RPM is set, in step 2 overheat the engine should idle down and then/or shut down.
The code may be "hard set" and will not clear itself in less than 50 'trips' [where the engine is cooled down, then warms up and then cools down again, not just key switch cycles].
His customer may be seeing the code at engine start up, and then it goes to the hour meter, as happens with "hard codes" that have been repaired but not cleared. "Hard codes" being those things the engineers/software developers think could cause damage to truck or injury to operator as opposed to those things that are warnings to emissions concerns.

DTC E-33 is Step2 Overheat signal
The engine coolant temp sensor output voltage has been less than expected voltage for a predetermined period (with coolant temp sensor normal)

Now for you to say the truck is running good with no problems is unusual, generally when the truck goes into step 2 overheat code it slows the truck down so it can cool itself. Maybe you just didn't notice the slow down or aren't concerned about it?
I would verify the trucks engine temperature physically using a handheld temp gun, check the coolant levels in the resivior and radiator. Go from there, if coolant low find the leak if it is leaking anywhere and have it repaired. Otherwise you may have a sensor issue or wiring harness issue at the sensor.