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Wouldn’t the state emissions system throw an error if a vehicle’s O2 sensor was reading 0/null?

Early OBD2 downstream O2 defeats were a simple astable timer (and, for some cars, a power resistor to simulated the heater draw).

The state testing in most states now is ‘make sure the ECU is reporting “READY” and has no emissions-related codes set.’

Since the advent of OBD2, it’s become rare for a state inspection to run the vehicle live for the purpose of emissions testing. Plugging in the OBD2 scanner is way faster, cheaper, more convenient, safer, and easier (albeit slightly) on the car than using a dyno.

Notable, however, California, the largest vehicle market by some margin, still requires sniffer tests.

* for vehicles model year 1999 and older, not more modern ones.

Only on older cars, which do not have OBDII. OBDI and before that require a sniffer. OBDII has enough sensors that the sniffer isnt needed. Am in CA, and only my foxbody ever got a sniff test. All of my other (newer) cars, the sniffer wasnt used

I cant speak for very recent systems, but they are not binary, the ECU expects a range of voltage readings...

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