§393.51 requires every air-braked vehicle to be equipped with a signal — visible and audible — that activates whenever the air pressure in the service reservoir drops to a level no less than 55 psi. The point of the rule is to give the driver time to stop the vehicle safely before the spring brakes apply at approximately 20 psi.
Severity weight + OOS
Severity weight 4 in the Vehicle Maintenance BASIC. An inoperative low-air warning is an OOS condition — the loss of the safety margin between full pressure and emergency application is the whole reason the warning exists.
How it gets cited
The test is straightforward: with the engine off, fan the brakes until the warning activates and confirm the activation pressure is at least 55 psi (typically by gauge). Common failures are a faulty pressure switch, a disconnected buzzer/lamp, or a switch set or stuck at a pressure well below 55 psi.
How to prevent it
- Verify low-air warning on every pre-trip — fan the brakes with the engine off and confirm both visual and audible warning fire above 55 psi.
- Annual inspection §396.17 must include the low-air test against a calibrated gauge, not a "the light came on" pass.
- Replace pressure switches on the maintenance schedule; the switches degrade quietly until they fail.
How Roadworthy HQ helps
A low-air-warning citation shows up in Roadworthy HQ as part of the inspection it came from, not as a record of its own: the app does not store individual violation codes. Log the roadside inspection on the vehicle's Inspections tab: date, state, level, driver, the number of violations, and whether it was out-of-service. For an out-of-service inspection the 15-day §396.9(d)(3) repair-certification countdown stays on that tab until you enter the certification date. The rule's 15 days cover every violation on the report, not only out-of-service ones, so for a non-OOS inspection that date is yours to track. The repair itself goes in the vehicle's maintenance log: date, type, what was done, who did it, and cost. DVIRs are logged per vehicle with the defect described in your own words, and a defect marked safety-affecting that is logged without a repair certification sends a critical alert. The §396.17 annual inspection report uploads to the vehicle, with reminders 60, 30, and 7 days before the next one is due.