Not certain about the FM but in the FH the sensors are under the OS corner panel. Open the grille and look behind, hopefully they will be there. Remove the corner panel to get to them. There is an issue with genuine Volvo sensors they fail after a couple of months, I use non genuine at present. If the sensors are not there let me know and I will ask. If it’s air gauge 2 it can be a dashboard issue. When showing zero give the dash a smack above the gauge on the top. If it comes back on it more than likely a faulty dash. These can be repaired ok.
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