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Would a faulty lambda sensor 100% show up on diagnostic?

Just wondering as my spark plugs are massively blackened when I take them out (even after being cleaned up only a few weeks ago) and fuel consumption seems quite high.

Took car to local garage and they couldn't get any fault codes on diagnostic check and gave a few reasons as to why I have these symptoms such as maybe burning oil due to age and condition of car, high fuel consumtion could be down to the weather affecting the cars operating temperature, so just wondered if anyone would have a different opinion or know of something else that could be causing my spark plugs to get like this?

My car is a w reg 1.4 SE 8v

Any help much appreciated, thanks

your garage is biased then as in cold weather the engine would run better as an engine needs cool dense air and would decrease your fuel comsumption. did you garage scope the sensor or was it just a quick scan and nothing?

As far as I know they just hooked it up to the computer an there were no faults.

You thinking lambda sensor could need replacing then?

Also would it cause any other problems other than that already described?

Thanks

could be a few things mate, like coolant temp sensor, lambda sensor etc