MHU45LP won't fire

blocked orifices at the two burners or weak pressure from valve.Are you running nat. gas or propane?

On - OK
1 Blink - Ignition failure. Check Gas flow, gas pressure, gas valve, flame sensor.
2 Blinks - Closed pressure switch. Check pressure switch (stuck in closed position).
3 Blinks - Pressure switch - failre to close. Check venter, pressure switch, vent blockage.
4 Blinks - Open limit switch. Main limit or aux limit are open. (My comment - Means overheating in chamber. Often result of blocked intake air. Bad filter. In my case, plastic wrap had been sucked into the intake. )
6 Blinks - False flame sensed. Check sticking gas valve.
Rapid continuous blinking - Incorrect twinning, incorrect primary polarity. Reverse transformer secondary leads, reverse primary wires.

Closed pressure switch can be from dirty combustion vents, bad chamber pressure - like missing screws, maladjusted gas pressure, failing pressure switch, loose hoses, moisture in the hoses or fan blades The switch need to be closed to ge the unit to work

Check the electrode is clean and sparking , clean flame sensor and check connections are tight . check hi and low limit switch . check pilot is not blocked .

clean the flame censor check connection from sencor is tight, try jumping out the limit switch high leval , or the pressure switch see if it lights then

The 3 flashes is an error code. Most likely you have either a vent blockage, the inducer motor is not spinning fast enough to draw a vacuum on the pressure switch, you have a bad pressure switch or a blockage in the hose going to the pressure switch. You can **** lightly on the hose going to the pressure switch to close it or you can jump it out to test it.

One blink does not tell you much, it tried 3 times to start and did not.

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).

Error Codes
A green LED indicates system faults
Steady On: System OK
Steady OFF: Board control fault or no power applied
1 Flash: Ignition failure (control in one-hour lockout)
2 Flashes: Pressure switch fault, fails open (switch is
stuck open)
3 Flashes: Limit switch open (main or auxiliary)
4 Flashes: Pressure switch fault, fails closed (switch is
stuck closed)
A yellow LED indicates flame status
Steady on: Normal flame is sensed
Flashes: As flame weakens, frequency of LED flashes
increases

Jump out the switch to test it. It could have a blockage in the line or it could be bad. Check the air intake and exhaust venting, there could be a blockage it it as well.

Sequence of operation, Gas Furnace.

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 ignitor. The ignitor will glow and you will be able to see it if viewed thru the small inspection port. Once the ignitor 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 ignitor does not energize (bad control, bad ignitor), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not lite (dirty pilot), the burner does not lite (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).

Most newer furnaces will have a diagnostic center or control built into the control module. You may be able to view it as it is running thru a small viewing port. Some models will require the removal of an access panel prior to finding it. On the reverse of the access panel, there will be a diagnostic chart that will aide you in understanding any error code recorded (usually a blinking light).

This is why we technicians earn the big bucks!