Stonster

Good question. I'd hoped that might be the source of the leak. But pushing the joint together made no difference, so I'm still trying to get to the bottom of it. A vacuum test should tell me if it's the pipework/ valve (cheap) or the servo (expensive). Or neither ( confusing!)

Agreed. It either seems like a leak in the pipework, non return valve, or servo. Why that should cause a fault code suggesting an electrical fault is another matter, but it would certainly explain the restarting issue

Right, update... I retested, and found I didn't have voltmeter connected correctly last time. Here's what I get: Input voltage to the sensor is about 5v Output voltage at ambient pressure (not running) is 3.9v. Output voltage at tickover is 1.4v Output voltage after running @2000rpm is 1.3v BUT after stopping the engine, the output voltage within about 10seconds increases again to 3.9v. Obviously without anything to compare it to, I can't tell if these values are correctly calibrated... but it certainly seems like the sensor is working. It also seems like I do have a vacuum leak. So checking that is the next step... (Quick question to check though - there isn't an electric vacuum pump at all is there. I can't see one, but thought I'd check...)

I did have to really dredge my memory / wikipedia to remember pull-up and pull-down resistors!

ah, yes, that would make sense. It might be that the ecu has a pull-up resistor and is set at 5.5v, but pulled down by a signal from the sensor. Then without a lower voltage from the sensor it remains at 5.5v which is what I measured it at. Sounds like the sensor might be faulty. More investigating needed, I think...

So this gets more and more weird... or I get more and more stupid! I've been into the plug and sensor and measured the voltages both with and without the engine running. Pin 1 is indeed a ground sensor Pin 3 is the supply voltage - stays at 4.9v volts regardless of what the engine is doing Pin 4 is the weird one. I would expect this to be some voltage less than the 5v input voltage, and to vary with pressure. If the sensor is not connected, I would expect 0v here. Instead its stays around 5.55 volts regardless of whether the engine is running or not, or even whether the sensor is connected or not. This puzzles me. Is there a reference voltage applied at the ECU? I'm just wondering if there could be a short at the ECU between pin 45 and (presumably) pin 44 or 46, causing pin 45 to be at 5.55v and not to vary with pressure? I feel like I might be missing something... So - does anyone have an ECU pin diagram? The ecu is behind the battery, right? Anyone heard of pins shorting before? It seems it bit far fetched to me but I'm struggling to understand the symptoms.

If you want to find the cover, it's here: https://www.ebay.co.uk/itm/SEAT-Mii-VW-UP-SKODA-CITYGO-2016-on-DASH-MOUNT-PHONE-HOLDER-BRACKET-1S0035935B/254242046120 You can get an adaptor to fit a later phone holder here: https://www.ebay.co.uk/itm/254369952197?ViewItem=&item=254369952197 and this bit as well: https://www.ebay.co.uk/itm/SEAT-Mii-VW-UP-SKODA-CITYGO-2016-on-DASH-MOUNT-PHONE-HOLDER-BRACKET-1S0035935B/254242046120?hash=item3b3201c0a8:g:qX8AAOSwEvRc9m2l But I wouldn't bother, just get a cover and a generic phone sat nav adaptor...

Well there's only one vacuum pipe to the brake servo... It's just whether the servo has a separate non return valve in it...

So frustratingly connecting the pipes more closely didn't resolve the issue. It's really weird - the way the sensor is acting (restarting the car after 20-40 seconds) suggests a drop in vacuum from a vacuum leak. But I can't feel this at all in the brakes. If there were a leak I would expect the brakes to get harder when the engine is stopped... But there's no feeling of this... Two thoughts: 1. Could there be a vacuum leak that's small enough not to feel at the pedal but big enough to set the sensor off? 2. Is there another non return valve in the servo? Then if I had a leak I'd only be losing vacuum in the pipe but not in the servo. Can't see why the system would need two valves though. I probably need to disconnect the hoses the check the non return valve, but I'm a bit nervous because they feel pretty stiff and I don't know how easy it is to connect/ disconnect them

Quick question: Can VCDS read and display live data from the brake booster pressure sensor. I'm having problems with this on my citigo stop-start and trying to find out if the sensor is faulty, the wiring is faulty, or if I have vacuum leak. Thought a live display of vacuum pressure would really clarify, but I don't know if VCDS will do that before I ask the garage to try it!

Will do. Just need a dry day to get out and play! Current plan is to gently heat the hoses with a hot air dryer to try to get them together more easily. I've also bought a second hand sensor to check the voltage values in comparison

Thanks mate - I can't see from your photo but are the plastic clips clipped together?

Following on from my search to try to solve stop-start restarting issues, I'm still trying to find the problem. I've checked the wiring to my brake booster pressure sensor, and the sensor itself was also replaced. Now trying to check all the hoses for a leak. I'm looking at one of the connections of the vacuum hoses and it looks a bit odd to me... I'm just wondering if this might be the cause of a vacuum leak? I'm just hoping a couple of folks might be able to send me a photo of their connection so I can compare. I had to hold my phone over it on macro mode, but it wasn't too tricky to get a clean shot. It's the connection to the right of the engine, near behind the air filter: My connection looks like this - are the black clips meant to be together?

It could be a vacuum or servo problem, I suppose. I've checked the hoses, but I don't think there's any way to check the brake servo

Yes, the sensor is the easiest fix, but least likely... Engine code is CHYA. There are 3 wires to the sensor. A brown one which I've checked to be earth, and two others. One should be 5v supply, I think, and the other the signal. But if you know where they end up that would be amazing!