Furnace sometimes will not stay lit

It sounds like the flame sensor is bad. If thats not the case you may need to replace the control board. You should make sure the flame sensor is directly into the flame.

The first thing I would do is make sure the flame sensor is clean, this is a metal rod immersed in the burner flame on the opposite side of the igniter. If too much oxidation builds up on it, it will not allow the burner to stay lit. To clean, just turn off furnace and remove flame sensor and clean with steel wool and then reinstall. If this does not solve the problem there may be a problem with the gas valve.

Hello, if the burners light but do not stay on for more than 10 seconds, I would clean the flame sensor. The flame sensor is a metal rod that is on the opposite side of the Igniter, sometimes it gets coated by oxidation and does not sense the burner flame and shuts the Furnace down. To clean to kill power to tje furnace, remove flame sensor and clean with a steel wool, then reinstall and start Furnce back up. If this does not work then most likely the Igniter is too weak to ignite the gas and will have ro be replaced.

I would have the ignition control board replaced. That is what tells the gas valve to come on and if there is flame or ignition it will keep the gas on if the flame sensor gives the board a signal that the burner is lite. By you shuting power off and on to the furnace it resets the board and it will go through its cycle again.

If your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then you need to clean your flame sensor with light sand paper or steel wool. You might need a new flame sensor, but most of the time they can be cleaned an will work well after cleaning.

Sounds like it is not proving flame, to check you need a meter that will read DC microamps. without knowing the brand I can't say what you should be looking for. Trane furnaces need at least 1 DC microamp to prove presence of flame. Usually you will see 2 - 2.6 or so, but it depends on what the manufacturer specs to prove flame. At minimum you can pull the burners out and clean them really good (especially the cross-over wings) because if the flame doesn't roll down to the sensor fast enough during the trial for ignition it will recycle and eventually lockout. If it stays lit longer than 20 sec, it most likely is proving flame and cycling off on high limit because the blower is not coming on. The typical blower on delay is 45 sec from the time flame is proved to when the blower is energized

Check the draft inducer air switch ( vacuum diaphragm switch) If the switch is opening when the burner starts, you might have a blockage on air piping coming into the furnace. Also prime the condensate trap on the condensate drain from the combustion fan. Also check the gas pressure, if your gas pressure is dropping, the gas pressure switch will shut it down. If it's too high the moment it comes on could raise the pressure in the furnace dropping out the combustion fan proving switch.

In the burner compartment opposite of the ignitor is the flame sensor. seems to be dirty. one screw holds it in. Remove and clean with a wire brush.

check the pilot gas pressure, might be too weak and blowing out when the blower starts, the clicking noise is the gas valve shutting down when it detecks no pilot flame, that park is normal.

If the burners do not fully ignite, the sensor will shut the furnace down. The lack of sufficient burner flame is a gas valve or gas pressure problem within the valve.
Replacing the gas valve with the original one may get you going. I surely suggest a professional service company for safety reasons and a thorough inspection. A 20 year old furnace is past it's normal life span.