Furnace is short cycling

[Ariu]

My furnace (Trane, 2 stage hi efficiency, installed in 2013) is short cycling.

Initially, when thermostat was calling for heat, the inducer fan was starting for +/-15 seconds and the furnace was shutting off without ignition. After 15-20 attempts and the inducer fan running at high speed the furnace would eventually start and was working OK until thermostat upstairs was satisfied.
I opened it up and the red light on the board was flashing 3 times meaning a pressure switch problem.
Last week I:
Replaced the filter
Checked air intake and exhaust and made sure they are not clogged by ice.
Checked pressure switches pipes for crack blockage etc.
Check pressure switch for continuity. PS was open no continuity.
Loosen up drain lines, sucked some junk and water out of it and turned the furnace on.
After doing all that, the furnace would need 5-6 attempts to start -much less than before and this time the inducer fan was running at a lower speed. Also this time the furnace would start occasionally for a short period and then shutting down. Red light still flashing 3 times.
I just removed the inducer fan and checked it for scale, debris, blockage. Nothing. The fan was clean.
I also removed and cleaned the flame sensor.
Put everything back and the same thing. Furnace is short cycling 5-6 times and then would start and work OK.

Questions:
Is anything else to be done, except replacing pressure switch?
Should I replace both (hi and low) or only the low pressure one.
Where can I buy them here in Calgary?

[graham1]

I’m not a furnace guy and without seeing your furnace my first thought is a flame sensor issue.
Pull it out and clean it with some scotch brite and pick up a new one ASAP.

[lmtada]

Here a good video. Good luck…high efficiency cycling, top four problems.
https://youtu.be/Gqoqa1nrWDc

[CBintheNorth]

If you jump the wires going to the pressure switch, does it function normally?
If it does, try replacing the pressure switch. They are cheap.
Do you have an AMRE there?

Our piece of crap York at the shop eats blower motors. Not necessarily the motor, but the squirrel cage. It cracks and opens slightly. The larger diameter starts rubbing on the housing, slowing down motor which in turn throws a code for the pressure switch.

[crazyperch]

Sounds like the air proving switch to me as well.

[CaberTosser]

Either the air pressure switch is defective, the venter motor is running at reduced RPM & needs replacement or something in the air pathway is blocked (even the secondary heat exchanger). Carrier once had problem secondary heat exchangers where a plastic liner delaminated and created blockages. I don’t know your model by heart, just noting possibilities.

Another issue can be blocked condensate drains/traps. I’ve seen it where the water was high enough that the inducer blower wheel was splashing through a pool of condensate and thus not getting to design rpm. Clear or replace the condensate trap in such instances.

[boah]

When you say “short cycling” do you mean everything works, but the furnace shuts off prematurely?
Or does the furnace not light the burner??? ( you may need to replace the igniter.)

[fishunter327]

I have seen issues with a blockage of the nipple on the vent fan housing where the pressure switch hose attached. A build up of deposits sometimes so hard they needed to be drilled out. Rarely the pressure switch was defective .
As Caber said in cold weather freezing or blocked condensate drain can be the problem.

[Ariu]

I took another look this morning and saw a section of drain pipe at the lower part of the furnace slightly bended and collecting water. I straighten it up and noticed quite a bit of water coming out. I suspect something in the air pathway is not right.
Anyhow, I am going to Amre supply to see if they have a switch for my furnace.
Thanks guys and marry Christmas.

[CaberTosser]

Blocked condensate traps can be more common in neighborhoods with lots of new construction or adjacent farming, etc as the dust gets drawn in & collects in the condensate trap, creating a muddy sludge that can completely plug up, creating a pool of water in the heat exchanger until the point it won't operate.

[mikevv]

Just a thought to check the exhaust coming out on your roof. Where I live there are 4 or 5 modular homes who have just about froze up due to their exhaust plugging up with frost and the furnace shutting down. It seems to only happen at -30 or colder.
Mike