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First, make sure the engine thermostat is working. Do this by starting a cold engine and holding the top radiator hose. It should be cold, As the engine heats up, the thermostat will "open" and the hose will become hot, as the water is allowed to enter the radiator.

Knowing you have hot engine water, find the fitting/hose at the rear of the engine, near the firewall. This is the hot water that waits to enter into your heater core, from the engine's manifold.

Next to this hose that disappears into the firewall, will be a second hose that exits and runs back to the side tank on the top of the radiator. This is the return hose from the heater core.

With the heat selector in "off", the return hose will be cool or cold. Selected to "Hot", the water will pass through the "now open" heater valve, through the core and back to the radiator. This makes that return hose warm/hot.

Turning the selector back to cool will close the valve, and the return hose will cool.

This is a non invasive method to check the valves ability to open & close IF the selector and it's control lines are working.

The valve may not be receiving instructions from the selector. Another problem entirely.

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Q: How do you troubleshoot the heater control on a 1994 GMC Sierra?
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