A three-way catalyst oxidises HC and CO and reduces NOₓ. Testing its performance is a matter of measuring how much of each it's removing and comparing against published conversion thresholds.
The conversion percentage
For accessible systems with pre-cat test ports (or estimated pre-cat levels from engine-out calibration):
// formula
Conversion (%) = 100 × (1 − post-cat concentration / pre-cat concentration)
Apply separately to HC, CO, and (where measurable) NOₓ.
Thresholds
- Healthy cat: > 95% HC conversion, > 90% CO conversion.
- Aged but legal: 75–85% conversion, tailpipe still passes MOT limits.
- Failed: < 75%, tailpipe limits exceeded, P0420 likely.
Tailpipe-only evaluation
No pre-cat port? Use the indirect method:
- Measure tailpipe HC and CO at warm idle.
- Compare to MOT limits (e.g. EU limits: CO < 0.3% at fast idle, HC < 200 ppm; exact values vary by vehicle year).
- Check temperature across the cat (IR thermometer) — drop from inlet to outlet = working chemistry.
When cats fail prematurely
- Persistent misfire depositing HC on the substrate.
- Oil burning (phosphorus/zinc poisoning).
- Rich mixture overheating and melting substrate.
- Silicone contamination from leaking coolant.
- Physical damage from undercarriage impact.
// fix the cause first
If you don't identify what killed the cat, the replacement will die too. Always resolve misfire / oil / coolant issues before fitting a new catalyst.