I have a Volvo S40 1.9T 2002, which my daughter uses and I do too occasionally. Have the following issues:


Part-1
1a) About a month ago as my daughter was driving on the highway the car stopped accelerating, the battery light and the check engine light came on, and she had to pull over to the shoulder and get the car towed. The car is now sitting on my driveway.
1b) When I try starting the car, there seems to be no compression.
1c) I checked the ODI codes and read P0335 (Crankshaft Position Sensor). Also there were two other codes P0341, and P0336 that were "Pending Codes" which my OBD code reader (Actron OBDII CP9125) manual explains as errors that have not appeared many times to register in the car's computer.


Part-2
2a) Also earlier (for over 2 yrs.) I had "Check Engine" light on, and occasionally (once about every 10 mins. of driving) the car would hesitate to speed up (for just 1-2 Secs.) when I pressed the gas pedal, but pick up speed immediately after. I got it fixed only last year.
My Volvo Specialist Mechanic read the error codes as P0237 and P0171 and replaced the Boost Pressure Sensor and after that it was working fine.
2b) Again about a month ago (which is about 10 months after the Boost Pressure Sensor was replaced), the car started doing the same thing - not speeding up (for 2-3 secs) when the gas pedal was pressed. But this was happening much less frequently about 2 times a day, as opposed to once every 10 mins. earlier.
2c) About a month ago I took it to my Volvo mechanic again, to see if the Boost Pressure Sensor had gone bad again. This time he came back with code P0420 and said I needed to replace Catalytic converter. Which I did not do, and kept running the car (after which Part-1 trouble happened).


3) So my questions are:
3a) Will it be OK if I just replaced the Crankshaft Position Sensor?
or
3b) Are codes P0420 and P0335 interlinked, and do I need to replace both Catalytic Converter and Crankshaft Position Sensor?
or
3c) Is it something else?

Some more Info:
The car has 170K miles, timing belt and water pump were replaced at 140K, but has been leaking oil from top gasket onto the timing belt. But
the timing belt is not broken.


Thanks in advance for your guidance!