Question 1

When a catalytic converter needs replacing

Accepted Answer

A catalytic converter changes harmful exhaust gases (carbon monoxide, hydrocarbons and nitrogen oxides) into harmless compounds using a precious-metal honeycomb structure. Over its lifetime the internal element can melt if there is unburnt fuel in the exhaust (faulty ignition), clog up with soot and ash, or be mechanically broken by an impact. Catalytic converter theft has also become common in Finland — a bare pipe in place of the converter produces an earth-shattering noise and won't pass the MOT.

A converter fault usually shows up as reduced engine performance, increased fuel consumption and an engine warning light with codes P0420 or P0430 (efficiency below threshold). A typical MOT fail comes from elevated CO or HC readings. Before fitting a new catalytic converter, however, we always run diagnostics — often the real fault is in the lambda sensor upstream of the converter or in the combustion inside the engine, not in the converter itself.

Question 2

How a catalytic converter is replaced

Accepted Answer

The car is raised on a lift and the exhaust is separated in sections. The catalytic converter's flange bolts are usually badly corroded and require heating or cutting. If the converter is welded inline with the pipe (common on older cars), the cut is made with an angle grinder and the new part is welded in. On modern cars the converter has flange connections, so replacement is quick.

The new converter is fitted with new gaskets and new bolts. Lambda sensors are screwed into the new part — seal integrity here is critical, as a leaking joint lets false air into the measurements and the engine management begins to over-fuel. After installation the engine is started and the diagnostic tool monitors lambda sensor values and converter operation in real time. A final emissions measurement confirms the MOT will pass.

Question 3

Why do catalytic converters clog or melt?

Accepted Answer

A converter melts almost always because of unburnt fuel that reaches the catalyst and combusts there, producing temperatures over 1 000 °C. This happens when ignition isn't healthy (worn spark plug or coil), when the engine runs rich from a faulty lambda sensor, or when the exhaust system leaks enough to add extra oxygen to the system. Converter clogging is a slower process: ash and soot particles accumulate in the channels through the honeycomb and over time the flow restriction grows enough that exhaust can no longer leave the engine properly. To prevent this, effective combustion — spark plugs, lambda sensors and fuel quality — must be kept in good order.

Catalytic Converter Replacement in Vantaa