Encountering a smog check failure with an “oxygen sensor not ready” message can be perplexing. This situation frequently arises due to the oxygen sensor, a critical component that communicates with your vehicle’s Engine Control Unit (ECU) via the OBD2 system, not completing its readiness checks.
A primary reason for this “not ready” status is often a recent ECU reset. When the ECU is reset, it clears essential long-term fuel trim data. Consequently, the engine management system relies more heavily on short-term fuel trims as the ECU works to relearn and re-establish long-term trims. This recalibration is crucial for accurate O2 sensor readings and overall engine efficiency.
If your OBD2 diagnostic tool or the smog check station indicates “O2 sensor not ready” after an ECU reset, performing a drive cycle is generally recommended. Driving through various conditions allows the ECU to gather necessary data, adjust fuel trims, and finalize the oxygen sensor readiness monitor. Allowing sufficient driving time is often the key to resolving this issue and preparing your vehicle for a successful smog test.
