S40: I have a Coolant Leak. Error P0420. Popping out of gear.

[mcfe]

I have a S40 02, 1.6 petrol only engine, 80kw, 109hp manual transmission, all leather, steering wheel on left if it matters.

This was my first car, I got it a few months back, after owning it now I am starting to see the issues and they are several, how severe I am not sure yet.

A few days ago I got a Check Engine MIL, using an OBD reader I found out it was code P0420. Upon further inspection under the hood, my coolant was on Min or lower! And I had it filled to max as well as changed oil 2-3 months ago. During that time I had traveled around 2000km or over 1200 miles. Although short trips, the longest being 100km or in the city.

Also since about 2 months now, when cold starting the car, while the engine warms up, the RPMs are rough, or rather the sound of the engine is sort of weird. The RPMs themselves are around 1000 but they oscillate by 50-100RPM before they stabilize in around 2 minutes to 750RPM. During this warming up period, I see white smoke in the exhaust, however this could just be water vapor due to condensation. However because my coolant is leaking somewhere, I am really hoping that's not what causes this rough warming up and the smoke/vapor and the reason for the P0420 code.
As for the oil level, I am not sure about that. I think it may have gotten a tiny bit lower.
Here is a video of the said vapor/smoke when I cold started the car in the morning https://streamable.com/7lg1z.

As for the gear. Well, I had a really bad experience a couple of days ago. I had to back out, and upon attempting to shift to reverse, it popped out...5 times. Now the thing is, I felt it wasn't getting in all the way. I tried depressing the clutch to see if it resolves it, it didn't. Only on the 6th try or so did I manage to get it in. Other than this, not sure if its normal, but most of the time when shifting to reverse, there is a grinding noise, even though I had pressed the clutch all the way and I have had the car parked and it was not moving. Not always, but around 80% of the time.

Another odd thing I noticed is that the fan turns on by itself for a few seconds and turns off, even though the knob is set to 0. The car isn't overheating, plus I've replaced the thermostat several months ago(and obviously thats when I refilled the coolant to max level).

[Dippydog]

How often and by how much are you having to top up the coolant?As a check most[if not all]garages should be able to do a "sniff test" to check for any sign of combustion gases in the coolant.

[mcfe]

As a "new" owner of this car, I only recently discovered this, so I do not yet have a solid idea of how much leaks per day, but I will be monitoring this situation.

[Notts Ian]

If you've only had to top up the coolant once it may be OK. It's usually necessary to top up after replacing a thermostat as a lot of air leaves the system in the first couple of hours running. Does your white smoke smell of antifreeze? That's often another clue to head gasket issues.

[pierremcalpine]

Don’t worry too much about the coolant leak. You need to fix it of course but it’s likely something simple like a cracked hose or cracked reservoir bottle. Take a good hard look at bottle and upper rear hose where it splits into two - those are two common points for leaks.

Some white smoke st start up is fine. White smoke while accelerating after car has warmed up...not so good.

As for the fluctuating idle if it’s a petrol start by giving the idle control valve a good cleaning. Easy job.