Does the heater work in defrost mode? If it blows out the same temp as the engine then your duovalve is probaly fine. I would get a used push button from Potomac German for testing purposes. The push buttons are very expensive.

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1986 190E 2.3-16 Pearl Black!!
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1985 500 SEL Anthracite Grey !!
2011 Subaru Impreza STI 380HP (Yeah Baby)