System problem. Customer stated that the furnace was short cycling. System would turn on for a min then cycle off. It repeats this over and over. This has been happening for the last few weeks. I checked filter. Filter was a bit dirty, but ok. Cleared drain line and checked drain trap. Cleared pressure switch tubing. No obstructions found. Cycled system and unit failed after a min with a pressure switch code. Removed intake venting pipe and cycled again. System ran for over 15 mins. Recommend furnace clean and tune. Recommend leaving venting off until cleaning. System operating at this time. Job complete.