After changing the oil in the car, 1 gear and second gear are useless. What happened?

After changing the oil first, the engine power will not be affected in this respect. Moreover, looking at the projects you have done in the future has little to do with the original failure, and it is likely that you have been cheated. Maybe there will be problems with these parts you replaced later, but let's deal with them one by one. The power problem of 1 and 2 gears has not been solved, obviously for money, so it is seriously suspected that the owner is not strong enough in the maintenance process. I suggest changing the repair shop and being as tough as possible.

This kind of fault is also a stubborn disease. It is necessary to check the vehicles one by one, focusing on the throttle, air intake system, exhaust emission, fuel quality, and the detection of engine computer (ECU). It is also possible that someone changed the data in the ECU, such as the initial fuel injection. Although this kind of speculation is a bit dark, it is sometimes inevitable.