As far as I understand, the ECU/TCM monitor multiple variables to decide if a shift command in manual mode is ok to execute. This includes engine RPM, readings from the MAF, speed at the wheels, etc. Now, for my actual question. What sensors does the car read when in manual mode? I'm still having issues downshifting (under 10k miles transmission/shift assembly), and want to go through all possible sensors to troubleshoot. My big suspicion is the crankshaft sensor, as AFAIK that measures RPM; if the car is unable to 100% determine RPM, it likely wouldn't execute a downshift. I've got fresh camshaft sensors, so that's already crossed off the list.