O2 Sensor - Bank 1 Downstream
2021 Toyota Camry · 2.5L I4
Sensor Type
zirconia
Waveform Type
dc-variable
Location
Exhaust pipe after catalytic converter, approximately 24 inches downstream of upstream sensor
Pins
4-pin connector
Idle Frequency
0.5 Hz
Timebase
2s/div
Voltage Scale
200mV/div
Trigger
450mV, crossing
Wiring
White (heater positive), Brown (heater ground), Black (signal ground), Purple (signal output)
Expected Voltage Range
At Idle
0.6V to 0.9V
At Higher RPM
0.1V to 0.95V
Expected Pattern
Lazy switching waveform, slowly transitioning between rich (0.6-0.9V) and lean (0.1-0.4V) with frequency much slower than upstream sensor
Known-Good Waveform
Traditional narrowband O2 sensor showing slow, lazy transitions with crossing frequency of 0.5-2 times per second during closed loop. Voltage should spend time at both rich (0.65-0.85V) and lean (0.15-0.35V) extremes with relatively flat tops and bottoms, indicating efficient catalyst oxygen storage. Transitions should be gradual, taking 200-500ms.
Common Failure Patterns
Fast switching similar to upstream sensor (more than 3 crosses per second) with quick sharp transitions
Cause: Degraded catalytic converter with reduced oxygen storage capacity, unable to buffer fluctuations
Voltage stuck at 0.45V or shows minimal movement (less than 0.2V range)
Cause: Failed sensor element or exhaust leak before sensor affecting readings
Diagnostic Tips
Primary function is catalyst monitoring, not fuel control. Compare switching frequency and transition time to upstream sensor; downstream should be significantly slower and lazier. Heater resistance should be 11-16 ohms. Perform catalyst efficiency test by comparing upstream and downstream sensor response during snap throttle test.
Related Diagnostic Trouble Codes
P0136P0137P0138P0139P0140
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.
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