Ammonia Slip Catalyst Efficiency Low

What this code means

The ammonia slip catalyst (ASC) downstream of the SCR is not effectively converting excess ammonia to nitrogen. Ammonia is escaping to atmosphere.

Symptoms

• Ammonia smell from exhaust
• Check Engine Light
• Failed emissions test
• Eye and throat irritation near exhaust
• White haze from tailpipe

Common causes

• ASC catalyst degradation
• SCR over-dosing DEF causing excess ammonia
• ASC contamination
• Exhaust temperature too low for ASC
• ASC catalyst physically damaged

Diagnostic steps

1. Measure ammonia levels downstream of ASC
2. Check DEF dosing rates for over-dosing
3. Monitor SCR and ASC temperatures
4. Inspect ASC for physical damage
5. Test downstream NOx and NH3 sensors

Repair notes

Replace ammonia slip catalyst. Correct any DEF over-dosing issue. Inspect SCR system calibration.

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