That’s true for many early OBD-2 cars, but anything reasonably modern (e.g. 2006) correlates signals between upstream and downstream O2 sensors. Most workaroun

That’s true for many early OBD-2 cars, but anything reasonably modern (e.g. 2006) correlates signals between upstream and downstream O2 sensors. Most workarounds you see on the market are patched ECU firmware to fib a scaled second O2 sensor value based on the first, so all of the diagnostic tests keep running. You’re seeing cars fail smog with these workarounds because states like California have started checking calibration IDs against the manufacturer’s listed values at the time of production.