I have a Bryant furnace model 398AAV updraft from 1992. Recently it will not start when thermostat calls for heat. Blower will come on when switched from AUTO to RUN so I do have 24v power. Self diagnostic goes through all steps OK. Will try to start three times then you have to kill main power to reset. Seems to stop at igniter heat. Igniter will glow for 30? Seconds then everything shuts down. Never seems to get gas. This unit does not have LED to show codes. Condensate lines are clear because when it does run water runs into pump. Could new igniter fix?

A faulty ignitor wouldn't cause immediate shut-down after only 30 seconds, but a faulty thermostat could. Try this: disconnect thermostat wires from furnace circuit board "R" and "W" terminals, and install a jumper wire connecting the two terminals to each other. Put the unit into operation - does it run continuously now? If so, something is awry with the thermostat. If previous problem continues, stop back & we'll have a look elsewhere. P.S. Do you have a voltmeter?

Sorry I have not checked sooner but do not have internet at home and off Monday. Did not see post until Tuesday.
I tried jumping R & W and furnace seemed to run fine this way. Stopped and started several times by opening blower switch. Started first time every time.
Went up to check thermostat and when I pulled off the switch plate I saw a loose screw on the base? And the wire was just barely touching the terminal. Tightened the screw and put wires back on the board in furnace. Furnace cycled on and off for the rest of the day OK.
Got up today and furnace was off again.
Yes I have meter and over the weekend I checked the following:
Have 25+ volts to limit switch and Flame roll out switch.
(Got these terms off schematic on door and measured while furnace was running.)
Have 25+ volts to Gas pressure sensor on gas valve and to collector box sensor.
Have 120+ volts to igniter during start up.
Pulled flame sensor was discolored so shined up with crocus cloth and put back. Did not know how to check for correct function.
Does any of this help?
Should the wire colors match the terminals? G for green etc. They don't on this board or thermostat. Could this be part of problem.
Will be getting new thermostat (programable) as soon as funds allow.
Thanks for help

Bunker762

Usually the first letter of the wire color is matched to the terminal with the same letter, but that is not always the case. Your system is color-blind, I assure you. As long as the wires are connected appropriately at both ends, color is irrelevant. You mentioned a gas pressure sensor - do you have natural gas or LP? Do you hear an audible "click" from the main gas control? Your "discovery" of the loose thermostat wiring was certainly beneficial, but I am starting to suspect a gas pressure issue.

I use LP (propane) and tank filled 2 weeks ago.
There is a click from unit after igniter glows but I have not figured out where it comes from. Board or gas valve.
Came home today and installed new thermostat and wire. Honeywell T874a1036. Furnace is worse now than with old thermostat. Bryant HH642501
When I can get it to light it shuts down 5 or 6 degrees below what the thermostat is set for.
The wife is getting perturbed about coming home to cold house and you know what that means.
Here are pictures of furnace and sensors.

Do not know if pictures are with text.
Have gas stove and waterheater and no problems with them.
There is some kind of pressure sensor on the incoming side of gas valve. It is wired in a loop with the collector box sensor. It has 25+ volts when induction fan comes on.
When I hear click there is 25+ volts to brown wire on gas valve. 8 to 10 seconds later the igniter stops glowing and the whole cycle starts again.

Had gas company come out and check for correct Pressure. Had no gas on burner side of gas valve. Cost 75 bucks to find out gas valve was bad.
Replaced gas valve and everything starts and runs OK.
How do I set heat anticipator on thermostat? Is it OK to set to same setting as old thermostat? Furnace seems to come on more than before but maybe I'm just paying more attention to it.
Thanks for your help
Bunker762

The anticipator should be set to match the control circuit current. Disconnect thermostat and connect an ammeter between "R" and "W". Wait for unit to progress to full operation (heating and blower on), note the value on the ammeter and set anticipator accordingly. I bet your wife is happier now...

Wife is happy this is over.
Do I run a wire from R to W and measure the current on that or just hook up probes? I have transferred the settings from old thermostat for time being. Electricity is not something I do everyday.
Bunker762

Put one probe to "R" and one to "W". If available, use a range/scale < 3 amps for greater accuracy.