At times my 2011 seinna willnot crank. the dash video will say that it does not recognise my key. then after a number of tries it will crank. my toyota dealer could find no problem.

You could try the spare key and you could read the vehicle handbook for advice.

Sometimes passive immobilisers develop amnesia and fail to recognise the number the key is programmed to and sometimes they break and need repair.

Sometimes disconnecting the battery overnight will restore matters.
One trick that has worked for me a few times is after the battery has been disconnected overnight is to insert the key and switch on the ignition (leaving the door open) before reconnecting the battery.

Try getting a friend to boost start your battery with his vehicle and see if your car will crank. If your car cranks over - you have a faulty battery.

Check your battery leads, particularly the black earth lead where it connects to the engine/bodywork.
It's worth pointing out that a failed battery will charge up to a certain extent - however there isn't sufficient power in that battery to crank the engine even though all your dashboard lights are illuminated.
Get somebody to try boost-starting your battery first.

If you're sure that your battery is ok (or if connected to another vehicle): Put your ignition key in and turn it on/off a few times. Leave it in the 'on' position for 2-3 minutes. Give the system time to re-recognise the key

It actually sounds to be battery problem
jump start vehicle Google Search

Engine not starting (does not crank) but all dash lights working ...

www.benzworld.org > ... > Mercedes-Benz Sedan Forums > W203 C-Class

Jan 18, 2011 - 10 posts - ‎4 authors
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Mercedes-Benz C-Class Questions - My car won't start, it ...

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CarGurus

Try this,,,,relearn steps are crank it ,leave the key on 5 min,turn key off and repeat.
sometimes takes 2 or 3 times to relearn

you need to access the trouble code to better define the problem. insert key into ignition. you will need to turn the key to the position just before the engine cranks; the dash lights will be on. now turn the key back one click. You need to do this a total of 3 times ending in the on position( dash lights on-not cranking). Your trouble code will either show on the odometer( if it's digital) or will blink on the check engine light. count the number of blinks-pause-blinks etc. jot the numbers down. You can google the numbers to see what system is at fault.

Pass key security system has (lost/failed too recognise key) try this; key out,key in,try too crank engine,release key leaving in run position for about ten minutes and/or till security goes out or remains on steady,this will relearn key if no other failures are present.good luck

be sure battery is charged check gauge on dash if it drops below 12 volts when you attempt to crank the car charge new battery also another quick test is turn on headlights if they go off when you hit the key battery is dead

All that advice seems correct. You only want to remove all the parts one time to get to the belt. So replace all the other things common to failure so you don,t have to redo the process