My Mk3 has just developed a fault on the rear parking sensors. Previously they worked fine, but in the last couple of days are emitting a constant tone when selecting reverse.
I’ve tried the WD40 clean, but to no avail. I’ve also looked behind the rear bumber for anything obvious, but all looks fine.
Can anyone help me with my next course of action? Can they be reset? Can I work out which, if any, are faulty?
Help…Thank you.
With the car in reverse, engine off obviously but ignition on, very, very lightly touch the centre of each sensor and you should feel a slight vibration. Check that they are all vibrating. Not the most scientific test but a good start.
Are they Mazda Genuine Accessory sensors?
I disconnected my rear sensors this year, those look aftermarket ones on your car. Mine had covers over the rear to stop muck getting in (factory fitted) yours may have dirty connections.
That bit of tape looks wrong, a bit of a botch-job. There could be water, dirt, something affecting the sensor or the wiring.
Find the control box and unplug each of the quartet of wires in turn, while the car has reverse selected ignition on and engine off.
Unplugging the dead one should stop the tone.
Now walk past them and the remaining good ones should respond with the tone as you go by, with the dead one silent.
This is where I put my control box, it is in the usual place on the right of the wide picture, as recommended by Mazda.
Detail picture showing it hiding behind fuel pipes
The sensors are usually the four on the left in this picture
I have the same problem. I have tested each sensor by removing one at a time as suggested, but the warning sound is still continuing. Could it be the control module or something else?
Thanks in advance
There are so many different types of control box on the market, but all looking much the same.
If you unplug ALL of the sensors and it still sounds, then the box is could be one of the better ones which sounds if there is any fault at all. This might help stop you reversing into the post behind a dead sensor - because you’ve fixed the dead sensor to stop the noise.
However this does not help with diagnosis.
As Roadster_Robbie suggested above for a first check, you might notice a finger-tip tingle when lightly brushing past a good one, and nothing with the dead one.
Also check each sensor for mechanical damage or dirt crust.
If you have some with a tingle, then the box is probably working on those outputs.
Swap round plugs at the box (eg shift all one step to the left) and see if the fault stays with a ‘dead’ sensor (sensor or it’s wire has the problem) or moves to another (the box has the problem).
There are usually two connectors in each sensor line, a supposedly “waterproof” in-line one under the bumper near the sensor, and the ordinary one at the control box. A problem with either of these having been disturbed is most likely. Eight connectors to investigate!
Having identified a non-tingle sensor, the swap trick can be tried again at this outside end to narrow down long wire or sensor, or connector.
Hello Richard
Thanks for your prompt reply.
I have unplugged ALL of the sensors and the warning sound is still activated. I think this must mean that the control module is at fault.
I think you are saying this, but there seems to be bits missing from your 2nd paragraph.
Please would you confirm if you think I am correct?
Many thanks,
Michael
Hmm, no. The second paragraph simply emphasises the pester value of the thing forcing you into fixing it sooner rather than later, somehow, anyhow. And I did then say it (the second paragraph) does not help with diagnosis.
Simple logic using permutations and the finger test to eliminate sections of the system is the best way to identify if fault(s) lie in box, sensor(s) or wire(s). It could still be any of these.
Least brain effort solution is to simply replace it all.
But myself, if it were my car, I’d search out the faulty bit(s) as an intellectual challenge, and then, probably, still replace it all.
For better access to the sensors and their local ‘waterproof’ connectors, take off the back bumper.
No fixings inside the boot, but the wires require removing the boot lining. The bumper can be slid back off the side mounts in about ten minutes IF two silly small bolts on the outside can be easily undone, those two unmentionables fixing it to the ‘captive’ square nuts in the bottoms of the wheel arch liners, which also need to come off. Good old Plus-gas is your best friend here.
Numberplate hides two screws so that needs to come off as well. Just six screws secure it to the body after liners and number plate removed. Don’t touch the side mount sliders.
A helper to support the awkward floppy bumper will make all this so very much easier. Less likely to damage the wires then.
I found the ‘captive’ square nuts can be held in place by a Visegrip. Here’s a pic of the driver’s side nut and some surprisingly clean thread of the bolt. But this still needs Plus-gas and wire brushing.
Hi, Did this issue ever get fully resolved, as I have the same issue on my Mk3.5 PRHT. It has cleared itself a couple of tines and worked correctly and not constantly beeping when reverse is engaged, which leads me to thinking its a loose wire/ connector?
I will do the things mentioned in this thread to see if I can find any thing obvious.
TIA
Mal
The reversing sensors on my 25AE (3.75 PRHT) didn’t work properly when I got the car. I tried all the options (WD40, tracing back the wiring and using Servisol to clean them, swapping connectors, etc) but it made no difference.
I ended up using these people (they cover Hampshire and surrounding areas). They came to me and replaced the whole system (control box, colour-coded sensors and beeper) for £160. Works perfectly now.
That’s brilliant Phillip!
Thank you very much.
I’ll give them a ring - not sure if they will come to Kent?
Best regards,
Michael
Thank you Phillip, I may well be giving Bob a call if I cannot find anything obvious myself😉
I think I have found the fault now, The sensors all of a sudden started working as intended and then failed again. So the control box and sensors are ok. This lead me to a possible wiring issue in the form of an intermittent connection. So, in order to narrow it down, I put the ignition on and gearbox in reverse and the annoying constant tone was there. So I tucked my head under the rear bumper and gently touched the wiring and at one of the sensors the nose stopped and by placing my other hand over the sensor in question it worked correctly.
Unfortunately the weather has been against me backing the vehicle up on the rams, do I can get right under and check the wiring and in-line connectors for connectivity issues. There is a break in the weather coming up tomorrow and Saturday, so may get a chance to finally cure the problem. I will keep you posted on this thread🤞