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In extreme cold weather it helps warm up the engine faster. Most of these cars also use a hot wire air mass meter between the airbox and the throttle body. The sensor works by measuring the amount of electricity required to maintain a preset temperature on a heated wire in the air stream. It also uses a built in thermometer to reference the air temp. The air flap mechanism in the volvo airbox works to maintain a minimum 70f intake temp in order to make the AMM readings more accurate. The problem with the volvo device is that its controlled by a wax filled thermostat device that pushes against a spring. When the thermostat fails (usually after 5-10 years), the air flap goes into hot air only setting. In this situation the intake temp shoots up to 180-200f, slowly destroys the AMM and increases NOx emissions by a factor of 10 and drops fuel economy below 20mpg. Most volvo mechanics know to replace this $5 item but more often than not, these cars end up in the junkyard due to failed emissions test.
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