I suggest the OP looks at the Toyota Yaris owners club website and searches for the MMT reset pages. There's some VERY illuminating information there.

Think you are right Repro .
The car is a 1999 yes 1999 on a Y reg.

Am I right in thinking this is a TC conventional gearbox ?

Would you recommend a rebuild at a auto gearbox specialist. We have one at Kettering Auto Gearboxes near us.

If the engine oil hadn't been changed for 22 years would you advise new oil without also replacing the filter?

Same for the gearbox. Drain, sump cover off, new strainer, new filter, new gasket, fresh fluid.

Slightly younger model, but same procedure.

https://youtu.be/kuYAPcvT83k

Really appreciate your advice.

Speaking to the specialist they have seen the electronic clutch causing these problems along with low battery voltage.

Has your specialist actually seen the car or is this over the phone? Your Yaris does not have an electronic clutch. Wonder if he is confusing it with a Micra or with the MMT box?

Edited by Rerepo on 25/01/2022 at 23:10

Not yet Rerapo . I’m waiting for my daughter in law to let me have the car . She uses it for work.

The auto specialist has branched out into EVs and also electric fork lift trucks.

Can you explain what components this Aisin gearbox has please ?

Cherrs

Torque converter, oil pump on front of input shaft. Epicyclic gear sets. Series of multiplate clutches and a brake band. I think there are three shift solenoids. Plus of course speed sensor, reverse/park switch and shift position switch.

I think it should drive in first and reverse. Assuming the shift selector cable is correctly adjusted etc and the box has fluid then the most likely problem is loss of line pressure due to internal leaks in the gearbox. After 22 years the seals are likely worn and hardened.

Edited by Rerepo on 26/01/2022 at 18:08