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BMW Catalytic Converter — Types, Failure, and Replacement
BMW Catalytic Converter — Types, Failure, and Replacement
What Does a Catalytic Converter Do?
The catalytic converter (cat) converts harmful exhaust gases into less harmful ones through chemical reactions:
- CO (carbon monoxide) → CO2 (carbon dioxide)
- HC (hydrocarbons) → H2O + CO2
- NOx (nitrogen oxides) → N2 + O2
Types in BMW
Three-Way Catalyst (Petrol)
Used on all petrol BMWs. Handles CO, HC, and NOx simultaneously. Requires stoichiometric air-fuel ratio (lambda 1.0) to function properly.
Oxidation Catalyst (Diesel)
Handles CO and HC. Used in conjunction with DPF and SCR systems.
SCR Catalyst (Diesel with AdBlue)
Selective Catalytic Reduction — uses AdBlue (urea solution) to convert NOx. Found on Euro 6 diesel BMWs.
Symptoms of Cat Failure
- Check engine light — P0420/P0430 (catalyst efficiency below threshold)
- Rotten egg smell — sulfur compounds not being converted
- Reduced power — clogged cat restricts exhaust flow
- Failed emissions test
- Rattling noise — internal substrate has broken apart
Common Causes of Failure
- Age — catalyst coating degrades over time (150,000-250,000 km typical lifespan)
- Oil burning — oil contaminates the catalyst surface
- Rich running — excess fuel overheats and damages the catalyst
- Physical damage — road debris, speed bumps
- Coolant contamination — head gasket leak introduces coolant into exhaust
Replacement Cost
| Model | OEM Cat Cost | Aftermarket |
|---|---|---|
| E46 325i/330i | €500-800 | €200-400 |
| E90 335i (N54) | €800-1,200 each | €300-500 each |
| F30 340i (B58) | €1,000-1,500 | €400-700 |
| G80 M3 (S58) | €1,500-2,000 each | €500-800 each |
Sport Cats vs Stock
Sport catalytic converters (200-cell) offer:
- Better exhaust flow (less restriction)
- Still pass emissions in most cases
- Typically used with downpipe upgrades
- Legal alternative to catless for road use
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