If you buy an ate sensor from a Motor Factors for an ATE system wouldn't you believe this to be a good OE spec sensor but this doesn't always seem to be the case, why is this?

Also why is it Bosch as an example list an oxygen sensor for some VAG cars where bosch is fitted as OE but the bosch aftermarket version from the same manufacturer doesnt work.

Also why do delphi list an aftermarket Delphi ignition coil for an Astra and an OE Delphi ignition coil which I'm asured is exactly the same spec/part although the OE is £30 more expensive.

Which route would you take knowing ignition components are critical but its your profit difference.