Bank 1 (driver side) exhaust pipe downstream of catalytic converter, approximately 18-24 inches after cat outlet
Pins
4-pin connector
Idle Frequency
0.1 Hz
Timebase
5s/div
Voltage Scale
0.5V/div
Trigger
Auto
Wiring
White (heater positive), Brown (heater ground), Black (signal), Gray (signal ground)
Expected Voltage Range
At Idle
0.6V to 0.9V
At Higher RPM
0.5V to 0.95V
Expected Pattern
Relatively flat voltage around 0.7-0.85V with very slow lazy oscillations (0.1-0.3 Hz) indicating catalyst oxygen storage capacity, much less active than upstream sensor
Known-Good Waveform
Post-catalyst narrowband O2 sensor should display relatively stable voltage between 0.6V and 0.9V with very slow, lazy oscillations of 0.1-0.3 Hz if catalyst is functioning properly. Voltage typically hovers around 0.75V with smooth, gradual transitions. The downstream sensor should NOT mirror the upstream sensor activity - if it shows rapid switching similar to upstream wideband A/F corrections, the catalyst has failed. During fuel cut, voltage drops to 0.1-0.2V indicating lean.
Common Failure Patterns
Rapid switching pattern mimicking upstream sensor activity with frequency above 1 Hz, or voltage oscillating 0.1V to 0.9V repeatedly
Cause: Catalytic converter failure - loss of oxygen storage capacity allowing unprocessed exhaust pulses to reach downstream sensor
Monitor downstream sensor activity versus upstream during steady-state cruise. A healthy catalyst dampens upstream fluctuations - downstream should be stable. Use intrusive cat test: briefly command rich mixture and watch downstream response delay compared to upstream. Response delay over 2-3 seconds indicates good cat function. Wrangler 3.6L cats typically fail around 120k-150k miles in dusty off-road conditions.
Related Diagnostic Trouble Codes
P0136P0137P0138P0139P0140
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.