Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
3-Way Catalyst Conversion Efficiency
A TWC requires a stoichiometric fuel ratio, 14.7 pounds of air to one pound of fuel (14.7:1), for high conversion efficiency. In order to achieve these high efficiencies, air/fuel ratio must be tightly controlled with a narrow window of stoichiometry. Deviations outside of this window will greatly decrease conversion efficiency. See Fig 1. For example, a RICH mixture will decrease HC and CO conversion efficiency, while a LEAN mixture will decrease NOx conversion efficiency.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.