Cause
Converter poisoned by oil or coolant consumption
A catalytic converter whose active surface has been chemically poisoned by engine oil or coolant passing through it. This is a distinct mechanism from thermal damage or fuel contamination, and it is worth separating because the converter is the symptom and the engine is the fault — a new converter fitted to an engine still consuming oil is destroyed the same way.
Phosphorus and zinc from engine oil, and silicates from coolant, coat the catalyst's precious metal sites. The coating is not burnt off by a hot run the way soot is; it is permanent, and efficiency falls progressively rather than suddenly. A converter that has degraded slowly over months, on an engine that uses oil or coolant, is poisoned rather than aged.
Find the consumption before quoting for the converter. Worn valve stem seals, tired piston rings, a failing turbocharger seal or a head gasket leaking coolant into a cylinder are the routes. Blue smoke, a dropping coolant level with no external leak, or oil fouling on one plug are the evidence — and a compression or leak-down test costs far less than the part it protects.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (4)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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