Mazda Forum - Mazda Enthusiast Forums - 2000 mpv codes p0506, p0740

P0506 means that the idle control system RPM is lower than expected. For this code to set the actual engine RPM at idle would need to be 100 RPM lower than target RPM for more than 14 seconds with brake pedal depressed and steering wheel straight ahead. This would also have to be detected by the PCM two different consecutive times for CEL to be turned on. Common causes would be a clogged air filter or otherwise restricted air intake system. Idle Air Control valve sticking or otherwise failing could also cause this problem.
P0740 is a transmission related fault code. It is indicating a malfunction in the Torque Converter Clutch system. When cruising at part throttle in fourth gear the torque converter "locks up" and mechanically mates the engine rotation directly to the transmission rotation much like a manual transmission with a mechanical clutch. When this occurs the engine and tranmission "should" should be rotating at very close to the same speed or RPM. If the computer sees that the RPM readings are significantly different then it is determines thatthe system is not operating correctly and sets that code.
Given the fact that your transmission operation is very erratic, jerking, hesitating, and also setting a P0733 code (gear 3 incorrect ratio), internal transmission failure is very probable with oil pressure loss being suspected..
Code could also be set due to failed torque converter clutch assembly or TCC solenoid valve fault but you would not see any of these other symptoms.