I thought looking at the MOT testers manual would help but now I am not sure?
But the fact its there at all suggests if the light is on = low tyre pressure = fail.
In 2021 I would have not relied on TPMS to be good enough to rely on, rather I used them for, "you have a puncture or your tyre is deflating" type of warning, perhaps there accuracy and reliability is much much better than I thought.
"Tyre pressure monitoring system (TPMS)
The inspection of the tyre pressure monitoring system (TPMS) is for M1 vehicles first used on or after 1 January 2012.
The inspection of the tyre pressure monitoring system (TPMS) does not apply to motor caravans, ambulances and hearses approved as M1 with a gross vehicle weight (GVW) of more than 2,500kg.
The TPMS warning lamp (see diagram 3) will generally illuminate and go off again when the ignition is switched on. If the system has identified a previous pressure loss, the lamp will remain illuminated. This does not automatically mean the system has a malfunction. In the event of a system malfunction, the lamp may flash a number of times and then remain on.
Warning messages on dashboard displays are not a defect in their own right, but may assist the tester in determining that the warning lamp is illuminated because a malfunction exists.
If it is unclear the warning lamp is indicating a system malfunction and not a simply indicating that one or more tyre pressures are low, then advisory information should be added to the test result.
Diagram 3. Example of a TPMS warning lamp
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