Driving with faulty dampers can throw the car off balance sending it out of control. Common faults of Magneto Rheological (MR) dampers are loss of oil volume because oil leakage. A fault detection system for monitoring the damper condition is proposed. The proposal monitors the transmissibility of the semi-active suspension of a Quarter of Vehicle (QoV) based on the frequency of the road. A frequency estimator for the road based on the deflection of the suspension is the key component of the fault detection system. In order to improve the efficiency of the detection system an observable domain switch was introduced. This switch identifies the most sensible transmissibility domain in order to improve the efficiency of the proposal. Intensive randomly generated tests with less than 36 % average in detection error using a semi-active MR damper validates the transmissibility as a good indicator of MR damper condition. © 2012 IFAC.