Some use the cars OEM Lambda sensor - if you car doesnt have one, then one will need fitting to the exhaust.
They are only as accurate as the sensor, and if they use the OEM sensor - thats not very accurate....
The expensive ones use a wideband lambda - these are accurate enough to tune with, but the price difference is huge...
One using a wideband lambda is £200+, one using a std lamdba £30ish.
TBH Id only use one if you planning on using it to tune the car, in which case spend out on a good one, if your checking for problems, compromise and get an EGT gauge instead at £80-£100
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