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Hi all,
Having an issue with stop start on a skoda citigo. I've had the fault codes read and it comes up with:
P0555 00 (032) brake booster pressure sensor electrical malfunction
P0556 00 (104)brake booster pressure sensor implausible signal
The stop/start system has a pressure sensor on the inlet vacuum hose between the inlet manifold and the brake servo. If the vacuum drops it's supposed to restart the car.
The fault codes suggest a sensor problem rather than a pressure leak...
So, I've been out with my voltmeter...There are 3 wires to the sensor.
Pin 1 is ground
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 ever 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...
Anyone heard of pins shorting before? It seems it bit far fetched to me but I'm struggling to understand the symptoms. The sensor was replaced just before I bought the car so I don't think it's a sensor issue. Equally it could be a vacuum leak but there's a different fault code for pressure too low/ high.
Any clever people with ideas? next step would be a vacuum test of the inlet to the servo, but I don't have a gauge so that would be a garage job...
Find out what it's supposed to do when the pressure changes, for example on my motorbike the workshop manual lists the correct resistance ranges for the ignition pickup, outside of that range won't work, no matter what you get on the multimeter. Without knowing what it's supposed to read under vacuum/non vacuum conditions makes it hard to know if it's fritzed or not.
I'd have thought that it won't be a variation, more of an on/off.
So if it's always live then it's stuck in one position and hence not working, again I'd want to know what it's set points are and what it should return in terms of stop/go signals, which is where the workshop manual comes in.
I’ve replaced numerous vac sensors, never had issues with wiring/ecu on that circuit. (VW master tech 17 years)
Sounds like might well be the sensor then... odd but not unheard of for two to fail in a row. Or my voltage measuring is not foolproof!
I’ve replaced numerous vac sensors, never had issues with wiring/ecu on that circuit. (VW master tech 17 years)
Out of interest, how many vac sensors have you changed, only to find it was a vacuum leak? Just would help to get an idea of what's most likely