Furnace short-cycling on cold mornings – Trane XV80, 12 years old

[hvacmafia]

Trane XV80, 12 years old, single-stage. Last 3 cold mornings (under 25°F) it’s been short-cycling — kicks on, runs for maybe 90 seconds, shuts off, kicks back on a few minutes later. Burns through a cycle every 4-5 minutes for the first hour.

Once it’s been running a while or once it’s warmer outside, it cycles normally.

Filter is new (1 inch pleated, MERV 8). Thermostat is a Honeywell Pro 4000, batteries new.

Is this a flame sensor thing? Pressure switch? Trying to figure out if it’s worth fixing or if I’m finally at “”replace the furnace”” territory.

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