O2 Sensor 1/2 Slow During Catalytic Monitor

What this code means

The PCM has detected that the downstream oxygen sensor on Bank 1 is responding too slowly during the catalytic converter efficiency monitor. The downstream sensor should show a relatively stable voltage if the catalyst is working properly.

Symptoms

• Check Engine Light
• Failed emissions test
• Slightly reduced fuel economy

Common causes

• Aging downstream O2 sensor
• Exhaust leak between catalytic converter and sensor
• Wiring or connector corrosion
• Deteriorated catalytic converter causing abnormal readings
• Fuel system issue affecting exhaust composition

Diagnostic steps

1. Monitor downstream O2 sensor voltage pattern on scan tool
2. Compare upstream and downstream sensor activity
3. Check for exhaust leaks near downstream sensor
4. Inspect downstream O2 sensor wiring and connector
5. Evaluate catalytic converter efficiency

Repair notes

Replace the downstream O2 sensor Bank 1 if it fails response time testing.

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