When calling for heat and turning the furnace on, the inducer and main blower start immediately. The igniter doesn't glow. Code implies high temp limit switch is open. I took out the temperature gauge that is juat behind the gas valve (in the plenum) and it shows 0 resistance.

Peck the high limit switch with a screw driver, it may be stuck. Or remove it and clean it off. How is you air filter?

I tapped the high limit switch and re-started. Inducer came on as normal and this time the igniter came on as well. However, there was no ignition. This is an igniter I replaced last October; however, the glow seems a little lame.

Is it a round ignitor or flat? I have seen the flat ones glow but not enough to light the gas. The round ones usually stop glowing at all. Inless it is one of the new metal ones which tend to not work when they are new.

I can't repeat it. I tapped it again and both blowers come on immediately. The igniter does not. Filter is clean.

Igniter is round.

You can take the wires off the high limit and jump them together. And see if it works. Usually the high limit will reset if tapped or cleaned. High gas pressure will cause the unit to not light, plus will cause the limit on go out if it does light

I'm not sure how to clean it. It looks like it is sealed. I tried jumping and got the same results. I cleaned the flame sensor and tried it with the bottom open, meaning no air restance to main blower.

Are you still getting a fault code 4? Does the ignitor light at all? What contol board do you have the 50a50 or the 50e47?

Just checked, still flashing 4 times. It is 50 a55 843, a universal board I replaced 8-10 years ago.

It could be a flame role out switch causing it. Look beside the burners for small switchs with red bottons in the middle. See if any of them triped.

There seems to be three of them on this furnace, one on the left, one on the right, and another one on the right, near the igniter. I am not sure how to tell if they have tripped. There is a small red "button" in the middle for which I could niether push in or pull out. Not sure what to do.

They should push in if they are triped. Sometimes they feel like they needed pushed in sometimes not.

I pushed them in as hard as could and started the furnace again. Both blowers came on immediately (as before) and after 30 seconds the inducer blower turned off.

Not being onsight I can not say for sure. But if the limitswtch is jumpered and all the flame role out switches are OK. The odds are it is your control board gone bad. Usually if the roleout or the high limit is triped the inducer will not start just the blower. Is this a 80% furnace of 90%