Transmission fluid pressure is crucial for smooth gear changes. Low fluid levels can lead to slipping gears and difficulty shifting.

Rainwater might be getting into a wiring harness or electrical connector related to the throttle actuator control system. This could cause temporary malfunctions during rainstorms.

While your mechanic initially thought the gear cables looked okay, they might be worth revisiting. Gear cables can fray or stretch over time, causing difficulty shifting. A closer inspection or even replacing them might be worthwhile, especially since it’s one of the few remaining original parts.

These sensors play a vital role in regulating the air-fuel mixture entering the catalytic converter. A faulty sensor, especially the sensor located after the converter (downstream sensor), could be sending incorrect signals causing the engine to run rich (too much fuel). This excess fuel can burn in the converter, generating extreme heat.

The age of the car is a significant factor. Knowing the specific year would be helpful to assess reliability. Generally, newer cars tend to have better safety features, fuel efficiency, and may come with remaining warranty coverage.

The alarm going off when you unlock with the key could be due to a malfunctioning fob or a problem with the car’s security system itself. A qualified mechanic can pinpoint the cause.

While you mentioned minimal rust, carefully examine the photos for rust around the fenders, wheel wells, rocker panels, and underside of the car. Rust can be a major repair cost.

Even though you mentioned minimal soot, there might be a slight blockage in the DPF causing excessive pressure and stressing the sensor. A thorough DPF inspection might be needed.