O2 Sensor - Bank 1 Downstream
2021 Honda Accord · 1.5L I4 Turbo
Sensor Type
zirconia
Waveform Type
dc-variable
Location
Exhaust downpipe after catalytic converter and turbocharger, approximately 24 inches downstream from catalyst outlet
Pins
4-pin connector
Idle Frequency
0.5 Hz
Timebase
2s/div
Voltage Scale
500mV/div
Trigger
Free run
Wiring
White (heater +12V), Black/Blue (heater ground), Black (signal ground), White/Black (signal)
Expected Voltage Range
At Idle
0.65V to 0.85V
At Higher RPM
0.1V to 0.9V
Expected Pattern
Relatively stable voltage around 0.6-0.8V with slow lazy switching, much less active than upstream sensor, indicating properly functioning catalyst oxygen storage
Known-Good Waveform
Post-catalyst O2 sensor shows minimal activity compared to upstream sensor, hovering around 0.6-0.8V with very slow, lazy transitions (2-3 second response time). If catalyst is healthy, this sensor should show flattened waveform with very small oscillations. During fuel cut on deceleration, voltage should drop to 0.1-0.2V and slowly rise back. Rapid switching similar to upstream sensor indicates failed catalyst.
Common Failure Patterns
Voltage mirrors upstream sensor with rapid switching (faster than 1Hz), similar amplitude
Cause: Failed catalytic converter with depleted oxygen storage capacity, unable to buffer exhaust pulses
Voltage stuck lean (below 0.3V) or rich (above 0.7V) with no movement
Cause: Failed heater circuit preventing sensor from reaching operating temperature, aged sensor element
Diagnostic Tips
Compare downstream sensor activity to upstream - should be significantly damped (lazy) if catalyst is functioning properly. Some 2018-2019 models had warranty extension for oil dilution issues which can damage catalyst over time. Use propane enrichment test to verify sensor responsiveness 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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