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Many a times, the dealer guy says that the unclean air filter consumes more fuel as compared to clean. So, my question is, is this really related to air filter? If yes, can we determine the case using OBD2 pid, that the air filter replacement or cleaning required for optimum mileage?

Thanks.

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    Fuel consumption with a dirty air filter is primarily a throwback to engines which are carbureted. In that case, it will go rich as vacuum increases through the venturi of the carb, more fuel is sucked into the engine.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Mar 1, 2016 at 1:02

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Modern gasoline engines have sensors to measure the amount of air that enters the engine. The ECU then injects fuel to give the correct mixture to get the most efficient burn. A dirty air filter will first show itself by limiting the flow of air at high throttle positions. Rather than causing a rich mixture however, the ECU will see the low air flow and inject fuel as appropriate. What you may notice, is a lack of power, not an increase in fuel consumption.

A diesel engine is lean burn engine and also has metering. The throttle determines how much fuel is injected, but once again, the ECU knows how much air has been taken into the engine and will not allow more to be injected than will give an efficient burn. So a dirty filter will only affect performance.

You could possibly use the OBD2 parameters to monitor the throttle position in a gasoline engine and compare it to air flow and determine over a long period how the dirty air filter is affecting air flow, but I can't imagine it being very accurate.

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