What happen when map sensor is worn out or faulty ?

Worn out map sensor may cause rough idle and increase in fuel consumptions - as the diaphragm rubber that measure the absolute manifold pressure getting weaker over time due to aging. As the ECU will received lag / delay data from the MAP sensor it will command wrong fuel delivery for combustions.

Faulty map sensor may result in engine difficult to start or start but having the worst idle as if the engine going to stop and it cause the ECU to command more fuel and this is the worst fuel consumption you can have. Significant weakness in the engine and feels like going to stop when on idle and you can smell fuel coming out from the tail pipe. 

While worn out map sensor can be tricky and difficult to detect but failed (bad) map sensor can easily be detected using cheap OBD2 data. For Perodua Alza the MAP sensor on idle ranging from 4 to 6.3 psi or more stable one around 5.8 ~ 5.9 psi but if it fails it will go up to around 14 psi pressure and you can feel the car run very weak with fuel smell coming out from the exhaust tail pipe and engine jerking to almost stop.