Madasafish

I have a Honda Jazz : recommended oil 0w20 For a year I used 5w30 - both fully synthetic.. (and both recommended grades) Over a 12 month period the difference in mpg was 0.4mpg on an average 43mpg.. (in favour of the 0w20). Hardly worth the extra money (These are Fuelly figures- dash computer figures are unreliable and mpg is so variable on style, terrain and temperature that the ONLY real test is over an extended period)

My son's 1.0 had fluid power steering..

If battery voltage is low you can get spurious signals...Suggest you sort out alternator/battery first.

From £20k used..https://tinyurl.com/y4fay6xd

Sounds like the connections to the guage are corroded/dirty. - probably at the tank end. I believe (?guess) you can access from the rear - under rear seat.

Yours is 2003 which is after the date when Yaris production for Europe moved to France. I am unsure if that included autos tho. Check your chassis plate under the bonnet and there may be a "Made in Japan" sign seen at the bottom middle of the front windscreen viewed from outside.. If your car is made in France - as opposed to Japan- it MAY have a different starter motor than that made in Japan. It is important you check: ...I doubt an auto vs a manual starter will differ but you never know. Be aware SOME parts suppliers - as opposed to Toyota dealers - do not have a clue about these possible variations...(been there)

Before you test anything check the earth straps connections at battery and body. All you need is some corrosion to drop the voltage . And then check the starter motor connections..

As the most likely bit to rust are the welds, I would use a penetrating spray.

The EGR valve may be carbonned up. Check that.

In my experience - and born out by scientific tests - Dintrol far surpassess Waxoyl Wax is easily washed away and requires to be diluted with thinners when spraying inside box sections or it may not penetrate rust but sit on top of it. Dinitrol makes specific treatments for external and internal use. The bottom sections of the Yaris were galvanised when new iirc but welding will destroy that at weld points.. The entire floorpan is galvanised.. hence the lack of underseal. But galvanising wears away with time if exposed to water and air- expect a 10 year life of exposed new metal..if galvanised.I don't think the box sections were wax injected from new...

Towbar bracket?

I treated ours long ago to prevent rust as the above listed causes - especially the filler pipe - seemed obvious (I wrote much of the Honest John guide to what goes wrong). I use Dinitrol. The rear sills at join in wheelarch need redoing...regularly due to stone and water damage.

I do happen to know what I am talking about, having experienced it first hand.

I had the same issue with ours.. Turned out to be glow plug failure.(easy to change) The bottoms of the plugs where they meet the cylinder head showed bad corrosion so I replaced two VERY carefully. That was false economy as the other two failed within a month.. Made no difference to ease of starting. EML on MOT fail.. I cannot recall how I tested them - I think I disconnected the rail joining all plugs and ran a wire to a single one and measure resistance to earth. One with the highest resistance (broken circuit) = failed... (I think) Or was it zero resistance? Sorry.. no idea.

20 miles at 5,000 rpm should clear it out...Or Yaris d4d failed mOT on emissions. Free retest 1 hour later after that showed emissions fell by 70% and it passed.. (and felt livelier too.Bit noisy though.)