Hello D Guilaume amd welcome to justanswer.com,
I not reall sure which gasket/o-ring they are talking about.
Did you take the Pilot into the shop for a specific problem?
Is this at the dealer?
oh ok once you said coolant leak I know exactly what they are talking about.
As you are standing in front of the motor the leak is form the right side of the motor.
There is a coolant passage that is there that runs to both cylinder heads. The main o-ring deteriorates and starts to leak coolant. Usually it is just a seeping of coolant and not an actual dripping leak, but I have seen a few leak.
The upper intake plenum has to be removed to do the job along with the coolant passage housing. the job can be a little involved. i can send you a link to replacing it if you wish to try it yourself.
here is the link you need.
I would say besides the water passage o-rings you will probably need to replace the upper intake gaskets and egr valve gasket.
here you go:
Make sure you have the anti-theft codes for the radio and navigation system, then write down the frequencies for the radio's preset buttons.
Disconnect the negative cable from the battery first, then the positive cable, and remove the battery.
Drain the engine coolant.
Remove the intake manifold cover and ignition coil cover.
Remove the vacuum hoses and breather pipe, then remove the intake air duct.
Remove the upper radiator hoses, lower radiator hose, heater hoses and water bypass hoses from the water passage, then remove the water passage.
Install the water passage in the reverse order of removal.
Install the battery. Clean the battery posts and cable terminals with sandpaper, then assemble them and apply grease to prevent corrosion.
Refill the radiator with engine coolant, and bleed air from the cooling system.
Clean up any spilled engine coolant.
Perform the powertrain control module (PCM) idle learn procedure, power window control unit reset procedure.
Enter the anti-theft codes for the radio and the navigation system, then enter the customer's radio station presets.
Set the clock.