2014 Smart Fortwo Knock Sensor Replacement Cost

Every time the engine knock sensor notices a vibration, it sends a signal to the engine's computer, reporting how large it was. The engine's computer is programmed to expect a specified range of signals from the knock sensor. Most information from the knock sensor will be bumps in the road or closing the car door. However, when a vibration is large enough, the knock sensor signal will trigger the computer to alert the driver. This is a protective measure meant to prevent complete engine failure.

When a knock sensor fails, the engine may lack power, have poor acceleration, ping or knock on acceleration, and get poor fuel economy.

The Check Engine Light may illuminate with a fault code set for one or more knock sensors.

When the knock sensor sends a warning signal to the vehicle's computer, the computer will usually reduce power output and illuminate the check engine light. Some vehicles will not restart until the problem has been fixed. » MORE: How to tell if you have a bad knock sensor

No. When the knock sensor sends a warning to the computer, it is always treated as if there is an internal engine failure. The vehicle should not be started or driven until the knock sensor is diagnosed as faulty, or the issue is repaired.

On most vehicles, the knock sensor is tough enough to last over 150,000 miles.

The check engine light and knock sensor-related OBD codes will immediately point a technician to the knock sensor. The technician will first conduct a visual inspection of the sensor and associated wiring. If the knock sensor wiring harness and electrical connectors are working properly, the technician will test the knock sensor. Further bench testing may be done. After testing is complete, the part will be replaced and the car tested to make sure the problem is resolved.

A knock sensor code isn't always triggered by a defective knock sensor. It can be triggered by a malfunctioning EGR system, a lean air-to-fuel ratio, worn ignition components, a cooling system problem, or an engine mechanical problem.

If the knock sensor is faulty, the technician will unscrew the bad sensor and install a new one. There is normally only one electrical connector per knock sensor, and there may be more than one knock sensor on the engine.