Heater does not heat up to max

check your heater core and your thermostat. your thermostat first.

It sounds like your low voltage circuit is open. Here are things to look for that you should be able to find on your own
1) check for a tripped limit switch

Please check rotate thermostat switch is put all the way to right side ( max Heat ). Switch on the heater. If the problem persists, then you can visually look inside, without opening the body of the heater, and see if the coil is getting heated up. It should turn red or you can feel the heat by placing your hand on the body. Also check if the fan is rotating. If the coil is not getting heated up then there is a loose connection in the switch panel. Take it to the retailer and ask him to replace it if it is in the warranty period. Otherwise you can open the body and check it your self if you have knowledge of electric connections OR take it to a technician

In some unit heat mode will start the fan after few seconds. Wait for max 3 minutes. If you turn it to fan mode, fan will only work no cool or heat will respond.

On the comfort zone CZ7007, the #1 rocker switch is for 600 watts, the #2 rocker switch is for 900 watts. If you flip 1 and 2 both to on you will get 1500 watts. Which is 600 and 900 added together. There is no down position, but only on and off.

make sure that you have adequate water flow going through the heater and that your water chemistry is balanced.

check pressure switch for proper adjustment and make sure filter is clean so system pressure is normal

Did you ever find a solution to this problem? My 2001 Sierra does the same thing; if I turn on the heat to full hot it remains stuck for a while. Sometimes it sticks for a couple of minutes, other times it sticks for a couple of days. As long as I don't turn it fully to hot though, it doesn't stick. It has gotten worse recently.

Hi!
There is a reason for your problem? have you replaced any components in the coolant system?
I/We need more information to help you and you make it very difficult for my colleagues to respond with the correct advice, a very short and brief description make it very hard for us to diagnose.
Please remember I'm sitting here typing all this for FREE!
I take all the lack of response on the chin as when the Fixya button is pressed I feel a sense of achievement! helping others in this Dark area of Technology.
So, if you've possibly upset your cooling system? you may need to bleed it as an Air lock could be the/ your problem
Set the vehicle on level ground, From cold remove coolant filler cap, start engine and leave to idle, turn heater blower on max and heat to full!
Have plenty of hot water available, keep clear of coolant filler! hopefully gurgling and a boil over will occur at operating temperature, wait! top up with hot water and wait again as it may do this a few times.
IF settled? and heat through interior then replace coolant cap and hopefully problem sorted?
If not! investigation in the cylinder head/gasket area is possibly required?
I do hope this helps?
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Thank you!
Paul 'W'
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