My heater stopped working on my Ford E-350 super duty, V10. Replaced thermostat and changed fluids. My gauge in cab stays in same place indicating no up and down temp.. When turn on heater no heat comes out. What am I doing wrong?

Not sure. Possible, Thermostat in backwards, Water Pump not pumping, or Air Blockage.

Is the engine heating up?
Let truck idle five minutes, feel radiator hoses.

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My gauge in cab stays in same place indicating no up and down temp.

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Indicates defective sending unit if engine is actually heating up.

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When turn on heater no heat comes out.

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Possibly a blocked heater core or lines.

Reverse flush the heater core.

Also make sure to bleed the air from the system.