Compare upstream and downstream signals, then investigate what the pattern can—and cannot—prove.
Explain the catalyst’s role in exhaust treatment; describe how oxygen storage can affect downstream sensor activity; compare upstream and downstream signals under controlled conditions; distinguish catalyst-efficiency concerns from sensor, exhaust-leak, and mixture faults.
Illustrative warm-engine model using conventional narrowband oxygen-sensor signals. Many vehicles use wideband upstream sensors and different monitor strategies. These traces, voltages, and diagnostic responses are not OEM specifications. Never condemn a converter from waveform resemblance or a P0420 code alone; follow vehicle-specific test procedures. Exhaust components can be extremely hot.
*Monitor eligibility is simplified. Actual enable criteria, readiness, thresholds, and code-setting logic vary by vehicle.
Top trace: upstream sensor (before catalyst). Bottom trace: downstream sensor (after catalyst). Illustrative 0–1 V narrowband-style display; not a diagnostic voltage threshold.