#catalyst
9 posts in Car Diagnostics blog about "catalyst" — OBD-II diagnostics, DTC codes, repair. ← All posts
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Bang on lift-off: exhaust backfire and what it means
A pop from the exhaust when you lift off is either an aftermarket calibration doing it deliberately or unburnt fuel reaching a hot catalyst. Those are very different.
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P2098 and P2099: post-catalyst fuel trim bank 2 lean and rich
Bank 2 versions of a code that catches people out, because it is about a correction the ECU makes after the catalyst — not about the catalyst itself.
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P0133: slow oxygen sensor response — the code that precedes P0420
P0133 measures how fast the upstream sensor reacts, not what it reads. Catching it early is what stops it turning into a catalyst code and a four-figure bill.
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P0138 and P0140: downstream oxygen sensor high voltage and no activity
The rear sensor does not control fuelling — it grades the catalyst. That changes what these codes mean and why replacing the sensor sometimes fixes nothing.
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P0421 and P0422: warm-up catalyst efficiency below threshold
A separate code for the close-coupled catalyst, and a separate meaning: this one is usually about how the engine behaves in the first minute, not about the converter.
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P2096 and P2097: post-catalyst fuel trim too lean or too rich
These codes come from the downstream sensor asking the ECU to correct the mixture. They are frequently mistaken for catalyst failure when the real cause is an exhaust leak.
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Exhaust modifications: what breaks besides the sound
Removing a catalyst or particulate filter changes more than the noise. The consequences for sensors, codes, and the annual test.
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P0430: catalyst bank 2 — why the "other side" of the engine fails separately and how to diagnose
Difference between P0420 and P0430, when both appear together vs separately, how OBD-II sees bank 2, why V-engines fail asymmetrically, and repair cost.
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P0420: Why the Catalytic Converter "Fails"
A detailed breakdown of code P0420: what "catalyst efficiency below threshold" means, common causes, how to diagnose it, repair costs, and whether you can keep driving.