ColinBarber

What is the error code, there are multiple things that can light up the VSC warning?

Welcome to LOC 🙂

Have you replaced your radiator? Certainly worth doing at least every 10 years to minimum the risk of the transmission cooler failing and allowing coolant into the transmission, completely destroying the transmission. I see this as a bigger risk than the transmission fluid if you had to do just one - but obviously worth doing both. Certainly more transmission problems reported on these forums are due to a radiator failure than lack of maintenance on the ATF.

Has the hood actually popped? If not then a false alarm and dealers should be able to clear for around £60.

Sealing up the transmission was a way to stop the fluid absorbing moisture as it essentially becomes a sealed system - the pressure valves being filtered. Manufacturer thinking is that period fluid change actually increases water content/contamination into the fluid, especially as if you just to a pan drain and refill you are only replacing 1/4 of the total fluid. Toyota/Lexus do recommend replacement every 60k miles if you are on a heavy maintenance schedule (taxi, only ever short journeys) but never if on a standard schedule. Wear is minimal, good for 200k miles on the original flu

I think you are going to suffer other issues long before you emissions go high due to carbon build up. Poor acceleration, poor starting, poor idle etc.

Not always, it has to be really bad for an error to be detected. I'd start with cleaning your MAF sensor as that is a simple thing to do.

have you ever had to change the O2 sensors? 99k miles and 13 years old, they might not be reading 100% which could result in higher than normal CO.

I've found the newer WLTP economy figures are much more realistic and easily achievable.

Premium Nav or standard? Never had an issue on my 2013 model with premium nav and an iPhone (not an X) but I'd normally use the wired connection as sound quality is better.

As the 400h has its battery in the engine bay it gets dirty and subject to moisture. Definitely worth cleaning it every year to stop a resistive path being formed across the top of the battery and adding to the drain. You can measure how bad it is with a voltmeter - I think I read over 1 volt once on my old 400h.

Leak in the heater matrix? Worth checking your coolant level. Otherwise you may have a windscreen that isn't 100% bonded.

yeah 1207B is the general one used on the engine. It is meant to ok with normal coolant but the long life variety can break it down over time. I think if you use it you will be ok for a number of years. I believe you are meant to let it cure for at least two hours before letting oil or coolant contact it.

Most tyre shops have Nitrogen if you want that instead of air. Tyre pressures are maintained for longer with Nitrogen.

Panasonic FR are more than good enough - high temp, high endurance, low ESR. 47μF - 63v - 13p from RS, problem is you need to purchase 200 of them 😞 https://uk.rs-online.com/web/p/aluminium-capacitors/9232827/ Farnell will sell you one, but is going to cost you over three times that of RS, at 41p. They have over 15,000 of them in stock! https://uk.farnell.com/panasonic/eeufr1j470/cap-47-f-63v-20/dp/2760347