The car will drive normally and this is rarely urgent, but it will fail an emissions test and the converter is not repairing itself. Before authorising a catalytic converter, ask three questions: has the exhaust been checked for leaks ahead of and between the oxygen sensors, has the rear oxygen sensor's response been tested rather than assumed, and what the Mode 06 catalyst monitor value is against its threshold. If a workshop cannot answer the third, they have not measured the converter — they have read the code. Also mention any history of misfires, oil consumption or head gasket work: a converter that was poisoned will poison its replacement.
The module has concluded that the catalytic converter on bank 1 is no longer storing and releasing oxygen well enough to be doing its job. It is among the most searched codes in the entire standard, and among the most expensively misdiagnosed — because the part it names is one of the costliest on the car, and it is frequently not the part that has failed.
Understand what is actually being measured, because the code does not measure the converter. It compares two oxygen sensors. The upstream sensor swings rapidly as the module cycles the mixture either side of stoichiometric; a healthy converter absorbs oxygen on the lean swings and releases it on the rich ones, so the downstream sensor sees a heavily damped, almost flat signal. When the downstream sensor begins to mirror the upstream one, the module infers the converter has lost its oxygen storage capacity. That inference is only as good as the two sensors and the pipe between them.
So there are three families of cause, and only one of them is a converter. First, a downstream oxygen sensor that has aged into laziness reports a signal that looks like a failed converter while the converter is fine. Second, an exhaust leak upstream of or between the sensors draws in outside air, which reaches the rear sensor and skews the comparison — a leaking manifold gasket, a corroded flexible section or a blowing joint all do it, and all are far cheaper than a converter. Third, the converter really has been damaged, and if so something else damaged it: prolonged rich running, a misfire dumping raw fuel into it, or oil or coolant consumption poisoning the substrate. Fitting a converter without finding that cause destroys the new one too.
The evidence that settles it is Mode 06. That mode reports the catalyst monitor's own test result against the manufacturer's threshold, as a number rather than a pass or fail — which tells you whether the converter is marginal or far past it, and whether it moved after a repair. A converter reading just under the limit is usually being dragged there by a lazy rear sensor or a small leak. One reading a fraction of the threshold is genuinely finished. That distinction is the difference between a sensor and a four-figure exhaust bill, and it is available on any scan tool that supports the mode.
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
The catalyst stores oxygen while it works. The rear O2 sensor exists to watch that storage: while the cat is healthy, the front sensor switches constantly and the rear one stays co...
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