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Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 22895
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Volkswagen polo 1.0 2001 engine management light keeps coming

Customer Question

Volkswagen polo 1.0 2001 engine management light keeps coming on, had the car on a diagnostic machine and its saying no faults, had the engine management light turned off but it comes back on hours later, ive changed the engine sensors that i could find but still the light stays on
Submitted: 3 years ago.
Category: Car
Expert:  Matt replied 3 years ago.

I'd 1st suggest checking that the engine earth connections are tight and free from corrosion , so check the main earth strap between the engine block and the chassis / battery earth and also check the small ring terminals that exit the engine bay loom and are bolted to the engine

the latter are the ECU specific earths and affect the sensor readings

if this is OK then I'd suggest getting a genuine VW diagnostic tool plugged in as sometimes the generic readers can be fooled by the odd code
Customer: replied 3 years ago.

already done all those things, it was a vw diognostic tool that was used, the people in the garage have no idea what it is, they haven't came along a fault like this

Expert:  Matt replied 3 years ago.
Hello Brett


in that case try cleaning the earth connections as I've suggested and also check all the connections to the engine sensors one at a time, remove each one and examine for signs of corrosion or damage and a squirt of contact cleaner ( not WD40) will also help.
Customer: replied 3 years ago.

the wiring is spotless, its all been checked, all the sensor plugs have been checked, ive changed the sensors on the engine just in case one was a bit faulty, there's been hours spent trying everything, the light stays off for hours even when its being driven then all of a sudden it comes back on, its proper doing my head in, is there a sensor im missing somewhere not on the engine, ive searched the internet and can't find anyone who's had the same problem

Expert:  Matt replied 3 years ago.
Hello Brett

can I ask what sensors you have fitted to your engine?

to my knowledge there should be the folllowing:

engine speed sensor (inside distributor)
throttle position sensor
exhaust oxygen sensor
air fllow sensor incorporating air temperature
coolant temperature

the 1 most likley to be at fault is the engine speed sensor as this is most susceptible to heat and vibration damage
Customer: replied 3 years ago.

i cant find the receipt off the garage, i thought there were 5 sensors they changed, but not sure, all i know is they changed every sensor on the engine and checked all plugs and cables to see if they were loose or damaged, it cost a fortune and in still no better off, someone said it might be a fault in the instrument panel, i just don't know but i need to get it done so i can sell it

Customer: replied 3 years ago.

the other thing i cant work out is if there's a fault why doesn't the engine management light come straight on after its turned of,

Expert:  Matt replied 3 years ago.
Hello Brett

as its takes a while for the light to come on its either a fault with the wiring so the fault is intermittant and needs things to expand in the heat

or its a isssue with the exhaust oxygen sensor / closed loop fuelling as this takes time to 'learn' and adjust the fuelling so if somethind is wrong and it can't adjust the fuelling far enough it will then flag up a check engine light

the other possibility is a faulty ECU

how does the car drive with this fault?
Customer: replied 3 years ago.

the car drives fine, fuel economy is very good, using more fuel is the first sign of the fuel oxygen sensor, plus that would have been picked up on the diagnostic, all obvious faults have been checked this is why its so puzzling