DSI-FLT comes on under two.. Or 3 conditions.
1: No flame detected when in GAS mode.. See footnote
2: Over temp (ECO) thermostat open due to over temp or failure
3: Wire discconnected (Red wire to ECO to be disconnected when winterizing) Or bad connection.
Footnote
When mine was new It often took 3 or more times before gas would heat the beast. Electric worked 1st time every time but with Gas I kept getting the Fault (DSI-FLT) Light... Finally gas would not work at all. New control board fixed it.
Now that's mine. do not know how common this issue is.