do you have the book sowing, could be a stuck blend motor, if memory serves me the system is always hot however a blend motor moves to alter the amount of cold passing through the heater, might be stuck in cold mode.
Also check the thermostat on the pipe to the heater matrix. This is one of the pipes accessible in the RH footwell by taking off the side panel next to your lower legs. This thermostat clips onto the pipe, and lets the HEVAC know the coolant is warm enough to deliver useful heat. Sometimes the clip breaks. I'm almost 100% certain you will find more detailed explanatory words and pictures on the main web site.
I don't think 'no heat at all' will be blend motors, although it 'could' possibly be a failed ECU (HEVAC).
What happens if you set the temperature to HI? This forces the blend motors to shut off the unheated air (so listen carefully, you should hear the blend motors moving the flap). ALSO manually turn the fan speed up, although a low reading from the thermostat may inhibit the fan speed request, it's not the sort of thing that is normally tested.
Thanks Chas I think yo have just explained a problem I have. I'll be driving along and the heat will be working fine on AUTO, then all of the sudden the fans will stop blowing, this usually only lasts for a 20-30 seconds or until I turn off AUTO and manually adjust the fan. So by your explaination it's probably the HEVAC or preferably the heater matrix water temp sensor or wiring.
I haven't had the chance to try and see if the problem has been fixed, however, I had to add quite a bit of coolant(it was near the botytom of the tank) to the "cold level here" mark so it could have been that since it was low, and I just had the truck idling, gonna try and try on monday and see.