A test is being done tonight (21 January) to see how quickly the tunnel fills with smoke in a fire.
It will be shut from 7pm until 6am tomorrow (Wednesday) to motorists and pedestrians..
Officials want to understand how long it would take to evacuate, and if any extra ventilation is needed.
Islanders are being told not to be worried if they spot any smoke during tonight's closure.
Another test is happening on Thursday (23 January), although the tunnel will not be shut.
The lights will be turned off from 9:30 am to 11:00 am.
This is so engineers can test the backup batteries.
The Tunnel from above the St Helier entrance/exit
The final set of tests will run from 27 January until 7 February.
Steel panelling is being removed from the tunnel to survey the concrete lining.
The last structural test was carried out a decade ago in 2015.
Motorists and pedestrians can use the tunnel as normal from Friday 31 January to Sunday 2 February 2025, and during the day Monday 3 February to Thursday 7 February.
Most of the tests will be done overnight to minimise disruption.
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