Sensors can fail but may not set a code because they are still within a range the computer believes is a valid reading. For example, a temperature sensor that normally reads -40 to +300 degrees fails but still outputs a reading of -40. The computer interprets this as a valid reading and prevents the transmission from shifting because it is too cold even though the ambient temperature may be +70. Same with a TPS that is giving a reading of say 30% throttle all the time. The computer simply believes you are cruising and doesn't set a code even though the TPS reading never changes.