O2 Sensor - Bank 1 Downstream
2021 Toyota Camry · 2.5L I4 Hybrid
Sensor Type
zirconia
Waveform Type
dc-variable
Location
After primary catalytic converter, approximately 30 inches downstream of upstream sensor, in y-pipe section
Pins
4-pin connector
Idle Frequency
0.2 Hz
Timebase
2s/div
Voltage Scale
200mV/div
Trigger
Not applicable for DC signal
Wiring
White (heater +12V), Brown (heater ground), Black (signal), Gray (signal ground)
Expected Voltage Range
At Idle
0.6V to 0.8V
At Higher RPM
0.55V to 0.85V
Expected Pattern
Slow lazy oscillation or relatively flat voltage around 0.6-0.7V indicating proper catalytic converter oxygen storage function
Known-Good Waveform
Post-catalyst sensor shows very slow lazy transitions or remains relatively stable between 0.65-0.75V, indicating efficient catalyst oxygen storage. Switching activity should be significantly damped compared to upstream sensor with less than 3-4 transitions per minute. Voltage should not mirror upstream sensor rapid switching; if it does, catalyst efficiency is degraded.
Common Failure Patterns
Rapid switching similar to upstream sensor, voltage transitions exceed 6-8 per minute
Cause: Catalytic converter failure with depleted oxygen storage capacity, common after 120k+ miles
Voltage stuck at 0.45V or below, no response to fuel trim changes
Cause: Sensor contamination from coolant or oil entering exhaust through head gasket leak
Excessive slow oscillation with amplitude greater than 0.4V swing
Cause: Borderline catalyst efficiency or exhaust leak between sensors affecting readings
Related DTCs: P0420
Diagnostic Tips
Compare upstream and downstream sensor switching rates and amplitude during steady-state cruise; properly functioning catalyst shows 80%+ reduction in switching activity. Hybrid thermal management causes frequent engine shutdowns affecting catalyst temperature; perform catalyst monitor drive cycle per Toyota specifications before condemning catalyst.
Related Diagnostic Trouble Codes
P0136P0137P0138P0139P0140
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.
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