Apologies in advance for what is going to be a long post. I want to be as clear as I can in setting out the background to my question.
Last January I had a 13 pin socket fitted to my Nissan XTrail as we had just bought a caravan. The socket was fitted by a specialist motor electrician. In March the car had its usual Main Dealer service and was MOT’d at the same time. It sailed through the MOT with no advisories. During the course of the year we toured with the caravan and the correct operation of all the lights was checked before each journey.

Fast forward to this year and the car was due its annual service, but I wasn’t able to book the MOT with them and had to arrange it elsewhere for the following day. When I went to collect the car I was surprised to be told that it had failed, the reason given was “Trailer electrical socket (13 pin) not operating the trailer lamps as intended (Stop lights inoperative) [4.10 (c) (i)]”

Now, I know that the stop lights on the caravan operate when connected to my car, but I accept that my word on this would not be enough, so I bought a Sealey TST22 test box (which is VOSA approved). This showed me that the stop lights are working.
I booked the car in for a retest and mentioned to the tester that my own observations and my (VOSA approved) test box showed that the stop lights were working. He showed me the test box he was using, which had 3 LEDs for the stop lights; 2 at low level, and 1 at high level. For my car, the high level LED was lighting up but not the bottom 2. He regarded this as a fail. He did agree to plug in my box, which only has 1 LED for the stop lights, and the LED lit up when the brake pedal was depressed.
As far as I am aware, pin 6 on the 13 pin socket is the one which controls the stop lights, so I am not how his test kit could isolate high and low level stop lights. I am also confused that 2 different pieces of VOSA approved kit appear to give 2 different results.

Any thoughts and guidance would be appreciated. My initial thoughts are to make sure I go back to the Main Dealer for next year’s MOT, as they have previously had no issue with this socket.

[b][user=786]Stealth[/user] wrote: [/b][quote]Hi Mack & welcome to the forum.

There is a known problem with some 13 pin socket test equipment. This is covered in the latest Special Notice issued by DVSA.

[url=https://www.gov.uk/government/publications/mot-special-notice-01-19-towbar-electrical-socket-test-tools-diesel-smoke-meters-and-annual-training-and-assessments/mot-special-notice-01-19-towbar-electrical-socket-test-tools-diesel-smoke-meters-and-annual-training-and-assessments]https://www.gov.uk/government/publications/mot-special-notice-01-19-towbar-electrical-socket-test-tools-diesel-smoke-meters-and-annual-training-and-assessments/mot-special-notice-01-19-towbar-electrical-socket-test-tools-diesel-smoke-meters-and-annual-training-and-assessments[/url]

[/quote][quote] It may be worth calling back & asking your tester to read the notice.Than[/quote]Thank you Stealth. I have been reading up on this issue, and lurking on this site gathering information. I was aware of the recent Special Notice and mentioned it to my tester. However, he did not think that this isue was related to the CANBus issues mentioned elsewhere as on his test box the LED’s were not flickering.

[b][user=19996]Paul S[/user] wrote: [/b][quote]interested to know the make & model of the trailer socket tester he was using, which had 3 LEDs for the stop lights[/quote]Hi Paul. I did ask before buying my Sealey as I thought there would be no point in buying the same model if it would only show the same result. He showed me it and it had no branding that I could see, just the Vosa approved Logo. He said that it was supplied by their Head Office, so all their garages use the same model.

I am still confused as to how his rig could differentiate between High and Low level lights when all the stop lights on the caravan are powered from Pin 6 on the socket.

[b][user=19996]Paul S[/user] wrote: [/b][quote]also confused any chance you can get a photo of their Vosa approved rig?[/quote]Probably not. I have never used this place before, and don’t know the tester that well. It’s a large chain, and if he’s correct in that they all use this type of kit, then there may be others who know what it is.