Bank 1 (passenger side) exhaust after catalytic converter, approximately 18-24 inches downstream of cat outlet
Pins
4-pin connector
Idle Frequency
0.2 Hz
Timebase
10s/div for catalyst monitoring
Voltage Scale
200mV/div
Trigger
Not applicable - analog signal
Wiring
White (heater +), Brown/Light Green (heater ground), Gray/Light Blue (signal), Black/White (sensor ground)
Expected Voltage Range
At Idle
0.6V to 0.9V
At Higher RPM
0.1V to 0.95V
Expected Pattern
Relatively flat voltage line around 0.65-0.85V with very slow, lazy switching (much slower than upstream), occasional gentle transitions between 0.5V-0.9V approximately every 5-10 seconds
Known-Good Waveform
Post-cat sensor should show minimal activity with voltage relatively stable around 0.65-0.85V if catalytic converter is functioning properly. Unlike upstream sensor, transitions should be very slow and lazy with switching frequency less than 1 cycle per 5 seconds. Healthy catalyst dampens oxygen content fluctuations, so rapid switching similar to upstream sensor indicates catalyst failure.
Common Failure Patterns
Rapid switching pattern similar to upstream sensor (more than 2-3 switches per 10 seconds), voltage fluctuating between 0.1V and 0.9V
Cause: Catalytic converter degradation or failure, no longer effectively storing and releasing oxygen, allowing fluctuations to pass through unmitigated
Use downstream sensor activity as primary catalyst efficiency indicator; compare upstream to downstream switching frequency ratio. EcoBoost engines with high oil consumption will eventually damage catalysts. Always verify sensor heater operation and check for exhaust leaks before condemning catalyst. Post-cat sensors have much longer lifespan than pre-cat due to lower temperatures and less contamination exposure.
Related Diagnostic Trouble Codes
P0136P0137P0138P0139P0140
Always confirm waveform data with your vehicle's OEM service manual before diagnosing.