When coupling a prime mover (tractor unit) to a semi-trailer, what is the purpose of conducting a 'tug test' (拉紧测试 / 牵引测试)?

1. To physically verify that the fifth wheel locking jaws have fully engaged and locked around the trailer kingpin before raising the landing gear
2. To test whether the engine turbocharger wastegate produces sufficient boost pressure
3. To calibrate the electronic tire pressure monitoring sensors on the trailer axles
4. To purge residual moisture and engine oil vapors from the pneumatic air dryer