Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Runflat indicator: Introduction
The Run Flat Indicator (RPA) monitors the tire pressure throughout the journey.
The entire vehicle weight is carried by the air pressure in the tires. If the pressure in one of the tires should drop, the tire will gradually "cave in". The resulting flexing means that the defective tire will quickly receive additional damage. The tire could burst.
Some 80 % of all flat tires are caused by small holes (e.g. caused by nails). The holes result in a gradual loss of pressure in the affected tire. Many "tire blowouts" can be traced back to preliminary damage caused by a gradual loss of pressure.
A gradual loss of pressure in one tire may remain unnoticed for a long time. The lack of pressure only makes itself clearly felt when driving when the tire is almost completely empty.
As tire pressure drops, the radius of the wheel and with it the tires rolling circumference will also decrease. The upshot is that the wheel speed of the affected tire is increased.
The RPA records the wheel speeds using the wheel-speed sensors of the Dynamic Stability Control (DSC). The RPA compares the speeds of the individual wheels and computes an average speed. In this way the RPA is able to detect a loss of tire pressure.
The RPA detects a drop in pressure below about 30 % +/- 10 % of the initial value. The RPA indicator and warning light indicates a drop in tire pressure. The RPA will indicate this after just a short distance, as a rule after a few minutes, from a certain minimum speed (e.g. 25 km/h) up to the permissible top speed.
Check tire pressures regularly, at least twice a month and before embarking on lengthy journeys.
During initialization, the set tire pressure is taken as the initial value for the current set of tires.
If all 4 tires lose pressure at the same rate, the wheel speeds will also change at the same rate. The RPA is unable to detect a uniform drop in pressure in all tires (e.g. due to diffusion = natural loss of air from all 4 tires)
As the emergency wheel has a much smaller diameter, the correct operation of the RPA can no longer be guaranteed.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.