I have a code 21 (gas lock out). The furnace starts and blower turns on. There is no gas. Igniter does not glow. Checked resistance across igniter and checked out at 68 ohms. Any ideas?

I hesitate to answer this question because of the dangers associated with gas--I hope you know what you are doing, or will find out before you do anything ;-)

I would suspect the flame sensor.

Take a look at the pages starting here: Troubleshooting an Electronic Ignition Gas Furnace - Overview and Repair (http://homerepair.about.com/od/heatingcoolingrepair/ss/furn_electr_ign.htm)

If you don't feel confident about tackling something like this, get some qualified help.