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Simple test to see why petrol gauge faulty?

Posted 29th Nov 2011
Members have kindly helped me out with several problems recently and so hope that someone can help with this.

A couple of mechanics have had a look at it and from suddenly one minute constantly showing full it now constantly shows empty .... but has not been fixed.

Very hesitant to go to another garage before seeing if there is anything I can test myself. e.g. fuses ? All the other gauges work fine.
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so one minute it works and the next it doesn't is that right? if that is the case it could be something simple like a loose electrical connection? I know that on my car there are some electrical connections under the drivers seat. could be worth unplugging them, give them a clean and plug them in again.
This could cost lots and lots of money to fix.
If its broke a mechanic can fix it if its intermittent its a lot harder to find and fix.
In a rover i had i changed tank sensor fuses bit of loom etc still not working.
The cheapest solution is keep it filled up.
Fill to the brim every time set trip clock to 0000000 the fill up when driven 300 miles
Trust me its cheaper than trying to fix it.
What car is it?

Sounds like a faulty fuel guage sender unit in the fuel tank.
Typically it's either a gauge is duff or the actual sender in the fuel tank has failed. Failure can be due to dodgy electrical contacts (clean/re-seat) or a failure of the actual gauge/sender itself (replace). A decent auto electrician will be able to do a quick continuity/resistance test on the wiring to narrow down the culprit.
Thanks for the feedback.

It is not an intermittent fault it is constantly showing empty .... prior to someone looking at it ... it was constantly showing full.
ask them what they did to it, sounds like they've just left a wire off
It could be the gauge. I used to have a Mercedes 190 and was able to pull the speedo cluster out and replace just the gauge.
You haven't mentioned what car it is but if there is a specialist near to you who only deals with your make of car, they may have more specific advice or, even better, the part itself.
The local MB independant specialist near us gave me a complete speedo cluster as it was such an old car. Still miss the car though...
Trying filling the tank and rocking the car....this worked for my sisters peugeot when it had a similar problem which was caused by her only putting a few quid in at a time.
I take it that its a Passat 1.8T 1999.

If you short out the pins on the sensor, the reading on the guage should show full. This will confirm the sensor being faulty.

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