I have a carrier weathermaker downdraft heat&ac unit that fails to open the gas valve soon enough after the ignitor is glowing before it cycles through another attempt or "locks out".
With the inducer motor running and ignitor glowing good either nothing else happens or a hissing noise like a servo motor comes from behind the top panel where the circuit board is before the click of the gas valve opening, but too late because the ignitor has "timed out" for the next attempt (or lock out).
If I restart the unit enough times it will finally open the gas valve soon enough to light and maybe continue automatic operation through the night heat demands only to fail the next evening after no demand during the day.

Attach meter to gas valve 24 volt terminals. Watch to see if 24 volts present when gas valve does not open. If 24 volts present and gas valve not open you have a bad gas valve or actually a slow responding gas valve and it should be replaced. Now if the 24 volts is not present or delayed then your problems are elsewhere.

The gas valve clicks open as soon as it receives the 24.7 volts. It just receives the power signal too late in the sequence to catch the hot surface ignitor still glowing (which has been glowing for approx 42 secs) to light the burners.
There seems to be a "pregnant" delay to open the gas valve?
I don't know what the hissing noise like a servo motor coming from behind the top panel where the circuit board is before the click of the gas valve opening is?
If the gas valve would receive the power signal as soon as that hissing event, it would be soon enough in the cycle to ignite the burners, I think.

It seems the circuit board is causing the problem since you say the gas valve opens as soon as it gets the voltage. I just love that term pregnant delay. That is a new one for me. Post exact model number and I will see if I have any other info since there are a few other controls in the command line for proper operation.

I am seeing a rash of this lately, both here and in the field. Try this: jumper the "R" & "W" terminals on the furnace circuit board & observe operation. A faulty thermostat may not be maintaining adequate control voltage under load, and introducing the gas control to the load could be exceeding the thermostat's ability.

KC13, I inspected the thermostat after I took the reading when the gas valve did open and was satisfied that it was just too late.

hvac1000, as far as I could gather the gas valve never failed to open once it received the power signal.

I got too single minded with the timing so I started to try to troubleshoot the circuit board.
I discovered I mischaracterized the "hissing" noise which was actually the gas valve relay "buzzing" before it positively closed turning the gas valve on so I swapped it out with the unused humidifier relay. The results were the same so I theorized there was a lack of adequate control voltage for the relay to "make". While inspecting the board I didn't like the presence of heat discoloration and corrosion on a few resistors (and was getting exasperated) so I ordered an ICM281 board from Patriot Supply.

Replaced the board and got ignition on first attempt.
Maybe next time I'll contemplate whether it was low control voltage or "retarded" cpu timing due to deteriorated components, or...

After the first successful ignition I had to swap the "standoff?" pegs from the old board that I forgot. When the next heat demand came on to prove normal operation the blower came on and stayed on and the board gave me a 33 flame rollout failure. WHAT NOW? I checked the limit swiches, cursed a lot, and thought about how nice a nursing home might be. Okay, so I forgot to plug the board back in the last time, I still know where I live, I think.

Thanks for the help.

What is the complete model & serial #'s of this unit? I hate to say it: I've seen the aftermarket board & don't care for it.

KC13,
I wasn't straight forward with my last answer. The ICM281 board from Patriot Supply is working perfect (so far). I just told on myself for forgetting to plug the limit switches terminal back into the board when I first allowed the unit to perform the final normal operation proof (It didn't know any better, that was on me).
The repair was successful. No problems now.