Found pressure switch opening, but inducer varying in speed. Removed intake and exhaust to rule out. Could then hear water and inducer. Cleared out water. A lot of junk in the collector box so flushed out. Ran on low for longer than recently per customer At about 30 PPMCO. Kicked in the high fire and CO started to climb to around 400-500 for a while and then peaked around 900. Let customer know heat exchanger is failing. Left unit shut down. Customer does have several CO alarms. wants price on furnace now and also price on doing furnace and AC at same time.