Had Trane 80% unit over 5 years old that developed an intermittent flame lockout after the first cleaning of the electostatic filter two weeks ago. The EFS was installed last June. Up until the EF cleaning, the 2 stage unit (which did actually use both stages) worked fine.

When checked out from the thermostat the unit would work fine. And the unit always started in winter mornings. Only randomly during the day would the unit lock out.

The regulator input pressure was 8WC and the stage 1 was 1.6wc and stage 2, 3.25wc both in spec. The hi out was .40wc.

The unit consistently failed on stage 1 when cycled by the Honeywell W8900 unit. That unit always cycles fan>stage1>stage2. Stage 1 would consistently fire only when the regulator stage 1 was set to 2.6wc, considerably out of spec. It always fired stage 2.

The final fix was an 8 cent part available at any local hardware store. Any guesses as to the problem? Questions welcomed.