350 MAV error code 24

[Mac220]

Secondary voltage fuse is open is what the manual says... what should I check?

Flashes a 21 when its running (Gas heating lock out)

comes on but no heat. The ignitor looks fine (how much resistance should I get?) No flame at all for the flame detector to detect The vacum pressure switch is new today, didnt fix the problem

[HayZee518]

check out the ladder wiring diagram for circuits derived off the transformer secondary. because the fuse is blowing, something is shorted out or is grounded. use the process of elimination for components - plug or unplug and try it.

[Mac220]

Still no heat

thanks Hayzee... When the furnace calls for heat the inducer motor kicks on, I get 24V to the pessure switch, when it builds pressure I get 24V on the other side. I do not get 24V to the Gas valve.

The blower motor kicks on right away.

Is the pc board bad?

[hvacwiz]

do you get 120v to at the ignitor? Did you repace the 3amp fuse at the circuit board? You should get between 60 to 100 ohms at the ignitor if you check the resistance.

[Mac220]

I checked the fuse right away with meter, its good.

The resistance in the ignitor is 150 ohms, It has been replaced by the previous home owner, I know this because they left the old one lay on the furnace. The one thats in there is not OEM, and is smaller type. It worked fine last winter.

I do not get 120V to the ignitor, I disconnected the ignitor and attached my meter (set to 200 VAC) to the leads going from the control board. Got about 10V

I do get 120V to the flame proving electrode.

[Mac220]

I need help determining if I should buy a new control board.

ITS COLD !!

I would think I would not get 120v to ignitor until gas valve opens.

[hvacwiz]

so your saying you get 24volts at the pressure switch ? when the inducer motor runs the pressure switch closes and you should have 0 volts accross terminals. if you still have 24 volts across switch it means you either have a blocked return or intake vent (birds nest maybe) a restristion in the pressure switch tubing a pluged condensate trap(if its a 90% furnace) If your blower motor kicks on right away then you have an open limit switch somewhere. Dont want to see you spend big money on a circuit board if you only have a 10$ limit open.

[Mac220]

Its a 2 prong switch, at the bottom I would say input side I get 24V when I first turn it on. If I put the meter on the top terminal it is 0 until the switch builds pressure I then get 24V there.