Many people ask why the Renault C565 catalytic converter contains two ceramic
monoliths:
The front section is coated with precious metals (PGM).
The rear section looks identical — but contains no precious metals at all.
So if the second one isn’t catalytic, why is it there?
Here’s the clear explanation:

1. Flow Conditioner

The rear monolith helps straighten and stabilize the exhaust gas flow.
This reduces turbulence and ensures that the PGM-coated section works at maximum
efficiency.

2. Thermal Buffer

It adds thermal mass and protects the active catalyst from rapid temperature spikes —
especially during cold starts.
This significantly extends the life of the PGM-coated monolith.

3. Coarse Particle Filter

Although it’s not a catalyst, it captures coarse soot and oil residues before they reach
and poison the PGM layer.
This prevents premature deactivation and clogging.

4. Structural & Acoustic Role

The “empty” monolith also helps stabilize the converter body, reduce vibration, and
improve exhaust acoustics and pressure pulses.

Why not fill the whole converter with precious
metals?
Because it would increase cost without adding meaningful performance.
Automakers use a short PGM-coated section + a longer support monolith, and still
fully meet EURO emission standards.

Bottom line
The second monolith is not a catalyst, but it plays a critical engineering role:
it protects, stabilizes, and optimizes the part that does contain platinum, palladium,
and rhodium.