My furnace will not start the ignition process unless I disconnect the pressure switch (combustion tube not blower tube). What could be the cause? It's a new pressure switch. The vent pipe is clean.

Are you referring to the combustion blower on your furnace? Cause if you are then maybe this answer will help. The hose coming from the inducer blower may have holes, and or the safety pressure switch is not working.

If the combustion blower is working the next step in the process would be the pressure switch. Check for plugs in the drain lines or venting. If the pressure switch is working it could be the ignitor.

sounds like its the pressure switch not working

If there is any restriction in the air intake pipe, exhaust pipe or the exhaust blower is weak; it will act as a pressure switch problem. I have found that if the venting allows the moist air from the exhaust to be sucked into the air intake, it will freeze and cause a no-heat until it has a chance to melt. Sometimes if the condensation hoses are not draining well, it will do the same as the pressure switch error code. Try disconnecting the air intake pipe and use room air for combustion and see if it starts on its own. If so, you have a restriction or weak blower.

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.

Make sure there is nothing in the flue or intake air pipe. make sure tubing to the pressure switch is clear and that the combustion blower motor is running. If all these are okay the air switch may be bad.

Sounds to me like the furnace isn't getting enough fresh air for combustion and the pressure switch isn't closing. try taking the cover off the burner assembly and c if that works

Have you tried to replace pressure switch for the combustion air. If your not sure where to find it if you pull the front panels off follow the flue pipe down to combustion chamber the combustion fan motor is either inside the flue pipe or out side of it not sure on the one you have but you should see two plastic tubes that connect to a barb onto the topside of the fan and one on the bottom they both should go back to the pressure switch. i hope this helps you if you have any more question or if this doesnt fix the problem just reply back to this message . Make sure fan is working when furnace starts up. It should be the first thing that comes on when there is a call for heat

1
1 FLASH
Furnace fails to operate.
• Integrated control module
diagnostic LED is
flashing ONE (1) flash.
• Furnace lockout due
to an excessive
number of ignition
“retries” (3 total)1.
• Failure to establish flame.
Cause may be no gas to
burners, front cover
pressure switch stuck open,
bad igniter or igniter
alignment, improper orifices,
or coated/oxidized or
improperly connected flame
sensor.
• Loss of flame after establishment.
Cause may be
interrupted gas supply, lazy
burner flames (improper gas
pressure or restriction in flue
and/or combustion air
piping), front cover pressure
switch opening, or improper
induced draft blower
performance.
• Locate and correct gas
interruption.
• Check front cover
pressure switch
operation (hose, wiring,
contact operation).
Correct if necessary.
• Replace or realign
igniter.
• Check flame sense
signal. Sand sensor if
coated and/or oxidized.
• Check flue piping for
blockage, proper
length, elbows, and
termination.
• Verify proper induced
draft blower performance.

You should be seeing 120 volts to the igniter. Anything less requires either a replacement of the transformer, start relay or control module.