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.