Update
The air cleaner intake hose arrived today. By following youtube's video instruction, I had no problem replacing the old one. After disconnecting the negative battery cable for a few minutes, the check engine light turned off. Now the car runs smoother than before.
Wei
Thank you so much for bringing up this topic and the detailed fix! My 2000 Protege ES has been trouble-free for 13 yrs until yesterday (148000 miles) when the check engine light flashed on. Following your advice, I checked the air cleaner intake hose. A huge crack was visible even from the top...