Catalytic Converter Failure
Catalytic converter failures typically fall into one of four
1. Thermal failure (overheating)
2. Plugged substrate
3. Thermal shock
4. Physical damage
Thermal failure is most often caused when excessive raw fuel
comes into contact with the catalyst, and "burns" in
the converter instead of in the engine. The high quantity of
fuel generates temperatures well in excess of the capacity
of the converter, causing meltdown of the ceramic monolith.
The melted ceramic could block the exhaust path, leading to
a significant loss of engine power. Visible symptoms include
heat-related discoloration of the converter shell.
Potential causes of thermal failure include: misfire, malfunctioning
oxygen sensor, fuel delivery issue, improper choke setting/operation,
and ECU malfunction.
A plugged or contaminated substrate can be the result of an
overly rich air/fuel mixture, radiator sealant, and oil or
antifreeze entering the exhaust flow. The resultant carbon
deposits restrict the operation - and ultimately the flow characteristics
- of the converter by coating the unit's reactive surface.
This degrades the converter's ability to perform its chemical
conversion process, leading to potentially illegal levels of
HC, CO, and NOx.
Root causes of this problem are a malfunctioning O2 sensor,
plugged or inoperable fuel injectors, piston blow-by, leaking
head gasket, broken or frozen choke or carburetor float, excessive
cranking time, and repeated incidences of running out of gas.
Thermal shock occurs when a fully heated converter suddenly
is "cold-quenched," such as coming into contact with
snqw or ice. This leads to sudden contraction of the converter
housing, which can cause cracks and disintegration of the ceramic
substrate. Symptoms include a "rattling'' sound when the
converter is tapped with a fist or mallet (monolith-type converters
Physical damage, caused by running over road debris, collisions
and other impacts, is usually easy to diagnose. This type of
damage can break up the ceramic substrate or cause restriction
that changes the flow characteristics of the converter or impacts
the efficiency of the catalyst.