This Bryant furnace stumped me. It sparks, pilot comes on and burners come on for about 20 seconds and goes off. I have changed the Flame sensor in front of pilot flame.
The Flame sensor has a white wire going to a Spark Power pack, and the red wire goes to the spark rod.
I get the same result after changing the Power Pack which also has 3 wires coming from gas valve. SOMETHING is shutting the gas valve down after 20 seconds.
I do know that on some other brands of furnaces that the Spark Rod also acts as a flame sensor. There's no model numbers left to be read other than the first 3 number "394".
I also changed the control board, and with all that... I still get the same results.
WHAT CAN IT BE?

I'm not for sure but I thought the 394 model was a standing pilot furnace. Can you send a pic of the furnace so we can see what you have?

I wish I could T-Top, but it was at a residence's place and it stumped me. The Bryant model starting with "394" is not a standing pilot. It has a double blower fans inside. You might say it's a fair size.
There's a THS, MV and a ground wire coming from gas valve to Power pack plus the sensor wire and the red spark wire.

To me it sound like it has a re-lite kit added to the system. If the flame sensor is grounded touching a burner or rust around the bottom it will do this, or if the spark ignition control is not properly grounded same symptom.

I replaced all new and nothing is touching anything... believe me, I racked my brain on this one and I have been doing this for 12 years. The only thing I can now come up with is that maybe a bad gas valve.

Hook your meter up to the gas valve terminals. Fire over the unit. When the flame drops out see if voltage is still being supplied to the gas valve. If voltage is still being supplied to the gas valve then you know the valve is bad.

Now if this unit has a 2 or especially a 3 wire pilot then the pilot warp switch is subject to be junk or someone might have replaced the glow coil setup with a spark setup. If the pilot has 3 wires suspect it. Unplug 3 wire pilot and jump yellow and green wire not on the pilot but the wires going back to the controls. This should keep it on constantly and if it is the 3 wire pilot then this test will tell you if it is bad or not.

Note do not try to adjust the safety pilot to make it work. That can turn into real bad news if it is not adjusted perfectly.

Note sometimes you can hit the pilot area with a screwdriver and the unit will start working again. This was always the quick test but unpluging and testing with a jumper is always best.