My laars 400,000 btu heater won't light up. The igniter glows but the gas doesn't start flowing. I measured the voltage across the gas valve and it measures 28v after the igniter glows. It goes through this cycle a few times then gives up. Any ideas?

Hi,
So you have voltage to the gas valve...
The question then is...does the gas valve open???
If you have access to a manometer you should place that on the "out" test plug or on the manifold to see if you are getting gas pressure or in other words gas out of the gas valve....that is assuming that you already checked that you have the right gas pressure coming into the gas valve...too high will lock up the gas valve. Anything over 14" WC and the gas valve will not open...
If you are getting gas to the valve and not out then you need to figure out the inbound gas pressure... if that is OK then the gas valve is bad...

If you have gas out and it is not going anywhere then there could be an obstruction in the manifold. Mud wasps do like lay their eggs in things like that and they can plug a manifold right off... they are a bugger to remove also...

So this should give you some ides of what to check for and what might have happened...

Comment back if this don't work...

heatman101