My mr heater big maxx turns off after 10 minutes of running. The only way to turn it on is to disconnect the pressure switch and once I do that it turns on but won't ignite. Once I reconnect the pressure switch it lights but then the process starts all over again. I replaced the fan motor and fan assembly and it started shortly after that. Any ideas?

Check out the pressure switch . and the snap disk .

Question edited for a lot more clarity.

From the maker.

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CK here......

https://www.fixya.com/support/t23702971-ignition_control_led_3_flashes

Best to you

replace motor

check the pressure switch and the snap disks

Your pressure switch may not be activating (ie closing and making contacts) This means you could have a blocked flue, clogged pressure switch tube or a bad pressure switch. The pressure switch is a safety feature to ensure your exhaust is not blocked and operating correctly. Find owners manual. Look at trouble shooting section. Open cabinet and look for error code light. If blinking 3 times, try "jumping" the pressure switch. To jump pressure switch you need to clear code be turning off power. When the unit/thermostat calls for heat the power vent fan starts first, this is when you need to jump the pressure switch. You need to do this in sequence. That is the unit first starts the power vent exhaust thus activating the pressure switch allowing the ignition cycle to the burners and heat. So the power vent fan needs to start first BEFORE you put the jumper on to bypass the pressure switch. Once the jumper is on it will call for ignition after approx 30 seconds. If ignites your problem is one of the 3 stated at the opening of this paragraph

is this a new heater or 2ndhand ???

the vent pressure switch will have a rubber type hose off it to the combustion fan housing. pull the hose off and poke the conector with a paper cilp or wire to make sure it is not plugged at the housing. make sure the vent fan is running and jumper the switch if it comes on replace the switch if not look else where for your problem.a volt meter is a great tool . voltage across a switch means it is open and no voltage means its colsed. i should say to also check your venting as it maybe blocked. there is no reset

Is the circuit board giving you a blinking error code?
On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).