Hope this helps someone else.

2004 Civic Sedan 1.7L Automatic with intermittent Engine Fan failure. Watched temps climb over 210F with BlueDriver App in Live mode.

Non-Honda Radiator Cap disintegrated. Replaced Cap and T-Stat with Honda parts. Burped properly. Engine Fan still inop.

Engine Fan, A/C Fan, Temp Sensor, Thermostat Fan Switch, fuses underhood, fuses under dash #14, fan relays, check out ok.

Are you sure?

I was only getting half the potential (6v) at the Engine Fan. Fan is a new one.

A/C ON only operates the Condenser Fan at this time. That's when 6V is found at Engine Fan.

9v Battery powered the Fan.

The A/C Fan connector powers the Fan.

Bypassing the T-Stat Fan Switch with thin wire jumper, Ignition power ON, hear the relay(s) clicking?

Probably not.

The Horn Relay is the same #Part Number Relay for the Engine and A/C Fans. Beep, it's a good relay.

I replaced the Engine Fan Relay. My car fixed.