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I have a York (Coleman) TM9X060B12MP11B furnace that is about 7 years old now.

For years I have had the issue that if it's cold outside, and often when it has been snowing, in the morning it kicks on, heats for a while but then stops.

The error codes flash 6 times, which means:

6 RED FLASHES: This indicates that after the unit was operating, the
pressure switch opened 4 times during the call for heat. If the main
blower is in a “Delay on” mode, it will complete it, and any subsequent
delay off period. The furnace will lock out for one hour and then restart.

Every time I have to wait an hour, then it will heat for a bit until the house is at temperature. Usually that's the last of it for the day, in the afternoon and evening it's fine.

When it happens I use a chimney cleaner to make sure no snow built up in the exhaust or intake pipes leading outside.

The furnace is positioned in the middle of the house with a run of about 40 feet leading outside (row house) with 2 small 45 bends to get it in the ceiling

The question I really have is, what does the pressure switch do? I see stories online about it triggering because the drain is clogged, but what drain would this be?

Does the pressure switch trigger because it can't get enough outside air in, or does it trigger because it can't get the exhaust air out quick enough?

  • I don’t know that model but many units have pressure switches on the heat exchanger a dirty filter or one with two high a Merv rating can cause a similar problem. Also dampers closed off or closed down two far. I have seen 1 exhaust pressure problem but dozens or more on the air handler side to prevent the fire box from overheating because of a dirty filter. Change your filter and possibly go to a “cheaper” one and see if that fixes your issue. – Ed Beal Apr 1 at 14:16
  • I have tried that yeah, I replaced the filter a few weeks ago with a MERV 11 rating. So is the issue that the furnace doesn't get enough air from outside? I was thinking to put a small booster fan in the inlet pipe, would that help? – Roger Far Apr 1 at 14:19
  • what Is the listed filter type ? This is a big deal I find where people keep putting bigger and better in but that is bad for the furnace, I use electro static and cheap spun fiberglass filters. I have had even my daughter call and they put in a fancy one hoping it would work better than I originally installed (just to have the same issue you are having). I am not saying it could be something else but only 1 time was that the case and it was a damper broke and blocked the air line to the combustion chamber , like I said very rare. – Ed Beal Apr 1 at 14:35
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    The pressure switch is usually related to the inducer motor--if it's getting weak it won't create enough pressure (vacuum) to maintain the switch state. I've had to replace inducer fans on a couple different furnaces I've owned. However, this usually results in the furnace not firing in the first place--not having it stop after it fires. – isherwood Apr 1 at 14:39
  • Inducer fan is a hard safety not a ok maybe. The exchanger pressure is more of a questionable as dirty filters cause the over heat 2 separate circuits on the units I have installed. – Ed Beal Apr 1 at 15:15

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