O2 Sensor - Bank 1 Downstream
2021 Hyundai Tucson · 2.5L I4
Sensor Type
zirconia
Waveform Type
dc-variable
Location
Exhaust pipe, Bank 1 (only bank on I4), approximately 18 inches after catalytic converter outlet
Pins
4-pin connector
Idle Frequency
0.2 Hz
Timebase
5s/div
Voltage Scale
500mV/div
Trigger
Not applicable for slow DC monitoring
Wiring
White (heater +), Brown (heater -), Black (signal ground), Gray (signal)
Expected Voltage Range
At Idle
0.65V to 0.85V
At Higher RPM
0.6V to 0.9V
Expected Pattern
Relatively flat voltage around 0.7-0.8V with minimal slow oscillation (0.1-0.3Hz), much slower and flatter than upstream sensor
Known-Good Waveform
Narrow-band zirconia sensor post-catalyst shows relatively stable voltage between 0.65-0.85V centered around 0.7-0.8V when catalyst is functioning properly. Signal should be much flatter and slower-responding than upstream sensor due to oxygen storage capacity of healthy catalyst. Very slow, lazy oscillations (once every 3-5 seconds) are normal. Voltage should remain fairly constant during fuel control changes that cause upstream sensor to switch rapidly.
Common Failure Patterns
Downstream sensor mirrors upstream sensor with rapid switching (0.1V to 0.9V) at similar frequency (1-3Hz), showing similar amplitude and response
Cause: Failed catalytic converter with reduced oxygen storage capacity, unable to buffer exhaust pulses
Sensor voltage stuck below 0.4V or above 0.9V with no movement, or extremely slow response time
Cause: Failed or contaminated sensor element, heater circuit failure preventing proper sensor operation, or sensor ground circuit issue
Diagnostic Tips
Monitor upstream and downstream sensors simultaneously to evaluate catalyst efficiency - post-cat sensor should be flat while pre-cat switches. Use snap throttle test: downstream should barely react while upstream swings full range. Check for exhaust leaks between sensors that can cause false P0420 codes on this platform.
Related Diagnostic Trouble Codes
P0136P0137P0138P0139P0140
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.
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