What is the HMF?

After clearing all my codes and driving for about a week I am coming up with the same code:

PIN 19:
4. Air injection system faulty hot film mass air flow sensor with hot wire.

I have searched this forum in and out to see if there is a fix for this problem. So far from the information I have gathered, other people who have had this code experience "surging" at idle. My car does this pretty consitently. I am pretty sure the problem is not the Air Pump as it sounds normal, the clutch engages and disengages, and the bearings seem fine. Could the problem be with my MAF Sensor or a bad connection to it? Thank you for all your help and information.

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• 1993 Mercedes-Benz 400E *146,000 Miles*
• 1985 Mercedes-Benz 300SD Turbo Diesel Biodiesel *SOLD @ 346,000 Miles*
• 1984 Mercedes-Benz 300D Turbo Diesel Biodiesel *SOLD @ 284,000 Miles*