Parking sensors

I have a 57-plate Ford Focus 1.6 Ghia five-door with nonfunctioning rear parking sensors. When I select reverse, there is no beeping, although the reverse light illuminates. I’m not sure if they are OEM or aftermarket sensors, but they are black in colour while the car is silver. I have carefully cleaned them and tried listening for any clicking noise, but I can hear nothing. Mike Boswell

The first point to establish is whether the sensors are factoryfitted or aftermarket items. The normal default function for a failed factory-fitted system is a continuous beep when selecting reverse. It can also be read for faults by a dedicated code reader.

The control box or parking aid module should be located in a space behind the rear offside wheelarch. Tracing back the wiring from the sensors to the control box, you should then be able to define if the wiring is a factory-fitted loom or an accessory connected into the reversing light circuit. Another giveaway is that aftermarket units have a built-in speaker, whereas Ford units have a separate speaker mounted near the rear seatbelt mechanism.

If the system is an aftermarket unit, the most cost-effective solution would be to replace the complete system; these are readily available starting from around £15 per set on ebay. If the system is an OEM unit and as it is not making any sound, the feed to the unit should be checked – this is pin number 1. The earth connection should also be inspected – this is pin 16. If the feed and earth connections are fine then the problem may be the speaker; this can be tested using an ohmmeter and the resistance should be 50 ohm + or – 7.5 ohm.