I recently went through the entire process of diagnosing a code PO401 on my 1999 Toyota Camry. I spent many days trying to locate the cause. I checked every component concerning the emissions control system EGR Valve, Vacuum control module, vacuum lines and everything else related but could NOT solve the problem. The very last component I checked was the Map sensor unit which is mounted on the fire wall just below the windshield. I disconnected the electrical connector and tested this part with my multi-meter and found this part to be OK as well. I reinstalled the MAP sensor and plugged it back in while being frustrated that I still could not find a faulty component and the check engine light was still shining bright yellow in my face.
Much to my surprise however, while driving the next day the check engine light disappeared. I drove for several weeks and the check engine light remained off and is still off today. All the while I was trying now to figure out why the light is gone and has not returned.
AND THEN IT CAME TO ME! My memory reminded me from my days of restoring vintage motorcycles that often times with electrical components, they simply fail to work because an electrical component that has spade connectors will become corroded or oxidized. The MAP sensor on my Camry is comprised of three flat spade connectors that connect to its power source and I think that my simple act of removing and reinstalling the power connector to the sensor caused it to start working again, thus turning off my dash warning light.
So I have learned from this that the very first thing to do is check not only vacuum lines for cracks, but also unplug and plug in any electrical components and check for corrosion on the terminals. I'm so glad that I did not just start assuming that major parts were defective and start throwing money at parts unnecessarily.
This made sense to me because of where the MAP sensor is located. It is on the back side of the engine where it is exposed to heat and moisture, the perfect recipe for corroded electrical connectors.