Catalyst System Efficiency Below Threshold (Bank 1)

This code means the PCM is noticing the catalytic converter is not doing its job. It can tell by the upstream oxygen sensor switching from rich to lean and back quickly to correct stoichiometry (that's its job) and when the converter cleans the emissions correctly the sensor after the converter should sit around a pretty steady 0.7 V indicating the oxygen level is correct. If the lower O2 sensor is acting the same as the upper and showing the rich to lean and back again — that means the converter is no longer cleaning the exhaust correctly and sets this code. You can see this in the data screen watching the oxygen sensor voltages in closed loop.

Downstream O2 (B1S2) voltage activity is tracking too closely with upstream O2 (B1S1). Catalyst monitor calculates the ratio and flags it once efficiency drops below the calibrated threshold.

Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.

1. 1Pull freeze frame and check for other active codes — misfires, fuel trim, O2 heater codes always come first.
2. 2Graph both upstream and downstream O2 sensors at 2500 RPM steady-state for 60 seconds.
3. 3Compare the waveforms. Healthy downstream stays flat near 0.7 V. Failed cat mirrors the upstream swing.
4. 4Check short and long term fuel trims. Anything outside ±10% — fix that first before condemning the cat.
5. 5Smoke-test the exhaust between the two sensors for leaks.
6. 6Only then: temp-test the cat. Outlet should run ~100°F hotter than inlet on a working converter.

• Worn or contaminated catalytic converter
• Lazy or biased downstream O2 sensor
• Exhaust leak before or between the sensors
• Engine running rich (kills cats fast)
• Misfire dumping raw fuel into the converter
• Aftermarket / non-OE converter

$200–$2,500 depending on vehicle. OE direct-fit on European vehicles can exceed $3,000.