Wideband is your air to fuel regulator gives you a accurate reading of those, the sensor gets welded into you exhaust not decat.. measures your readings from there, FCD is basically stops the car from hitting fuel cut when you up the boost, wouldn't bother with one just buy a plug n play ecu or a mappable ecu FPR it regulates fuel at a constant set pressure we already have one on as stock but people upgrade them for a more accurate fuel pressure The only one worth buying is the wideband imo but everything all depends on what mods you want in the future.