Question about 1991 Cadillac DeVille

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My 91 cadillac devile has no heat , just cool air blows even when

Even whenit set on 90 it blows cool air.

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The heater in your car depends upon a flow of hot coolant going through the heater core in order to get hot air into the car. If the heater core is partially plugged, it will restrict the flow. If you have a head gasket that is putting exhaust into the cooling system, it will eventually get into the heater core. When this happens there is no coolant in the heater core to make the air warm. If the water pump vanes have corroded away the result will be very poor coolant circulation causing the heater to not blow hot air. If your car is equipped with a heater control valve that is not functioning properly, the coolant flow will be restricted. One or a combination of these factors is likely the culprit in your case.

if the car is not overheating, the thermostat is working fine.... replace or flush the heater core multiple times to try and eliminate sediment build-up.

Posted on Jan 14, 2009


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Posted on Jan 02, 2017


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The heater and A/C programmer supplies voltage to an internal motor that drives the air mix door. The heater and A/C programmer monitors the air mix door position feedback from a sensor on the motor. If the air mix door is requested to move but the feedback voltage does not change, the heater and A/C programmer will store DTC B1340. If the air mix door feedback indicates that the feedback is near the hot or the cold extreme, the DTC will not set. Evaluate the operation of the air mix door through self diagnostics as described in the test description.
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Did you perform the Diagnostic System Check?
Go to Step 2
Go to Diagnostic System Check
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Is the door being commanded (COM) to the proper high and low values?
COM High 100 %
COM Low 0 %
Go to Step 3
Go to Step 4

If you are unfamilier with this , i suggest you take it to a ASE certified repair shop or the dealer .

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