O2 Sensor - Bank 1 Downstream
2017 Toyota Camry · 2.5L I4
Sensor Type
zirconia
Waveform Type
dc-variable
Location
Exhaust pipe after the catalytic converter, approximately 24-30 inches downstream of upstream A/F sensor, monitors catalyst efficiency
Pins
4-pin connector
Idle Frequency
0.2 Hz
Timebase
10s/div for catalyst monitoring
Voltage Scale
200mV/div centered on 0.5V
Trigger
N/A - use DC coupling
Wiring
White wire (heater positive), Brown wire (heater ground), Black wire (signal ground), White/Black wire (signal output)
Expected Voltage Range
At Idle
0.1V to 0.9V
At Higher RPM
0.1V to 0.9V
Expected Pattern
Slow, lazy oscillations with much lower frequency than upstream sensor, should remain relatively stable near 0.45-0.55V with minimal switching if catalyst is functioning properly
Known-Good Waveform
With a properly functioning catalytic converter, the downstream O2 sensor should show very slow, lazy oscillations staying primarily between 0.4-0.6V with minimal activity - much less responsive than the upstream sensor. The sensor may show occasional slow transitions between rich (0.6-0.9V) and lean (0.1-0.4V) but should not mirror the upstream sensor's activity. Typical frequency is less than 0.5 Hz with the voltage spending most time in the mid-range, indicating the catalyst is buffering oxygen storage and release.
Common Failure Patterns
Voltage mirrors upstream sensor with rapid switching (1+ Hz), following same pattern as upstream A/F sensor
Cause: Failed catalytic converter with insufficient oxygen storage capacity, allowing exhaust pulses to pass through unmodified
Voltage stuck at lean (below 0.2V) or rich (above 0.8V) with no switching or very slow response
Cause: Contaminated sensor (same causes as upstream), failed sensor element, exhaust leak before sensor affecting reading
Diagnostic Tips
This is a conventional narrowband zirconia O2 sensor used exclusively for catalyst monitoring. Compare its switching frequency and voltage range to the upstream A/F sensor - healthy catalyst should show 60-80% reduction in switching activity downstream. Use propane enrichment test to verify sensor responsiveness before condemning the catalyst.
Related Diagnostic Trouble Codes
P0136P0137P0138P0139P0140
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.
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