Thanks for giving me the opportunity to help! The fact that the inducer motor is running, but the unit won't go any further could signal:
1) bad pressure switch
2) kinked or disconnected pressure switch tubing
3) plugged intake or exhaust (you didn't give me a model number so I don't know if this is an 80 or 90% furnace-full model number would be helpful)
4) slow or defective inducer motor
Yes, I understand the suggestions you've made. Those are the ones I've seen from my previous internet research.
I've removed the ends of the tubing from the pressure switch. And, I placed a suction on the tubing and heard the click from the switch. However, the suction I'm placing on it may be more than usual and not be a good diagnosis.
The model number is a EnviroPlus90.
The model number is GUK100N16C1.
The tubing isn't kinked or disconnected.
The exhaust appears to be clear.
Is this a bit older furnace? If so, these had issues with a crack on the secondary heat exchanger if it was never replaced. Sometime the secondary would even plug up with soot. Check your primary heat exchanger and look for any signs of soot. Unfortunately, inspection of the secondary heat exchanger is quite a job and probably best performed by a qualified HVAC technician.