P2423: HC Adsorption Catalyst Efficiency Below Threshold Bank 1
What to do
The car will drive normally — this device only does anything in the first minute after a cold start, so there is nothing to feel. It will fail an emissions test. Before agreeing to replace it, ask whether the oxygen sensors have been verified, because the efficiency judgement is calculated from them and a drifting sensor condemns a healthy exhaust component. This is not a repair to rush into on the strength of one code.
What this code means
The hydrocarbon adsorption catalyst on bank 1 is not storing and releasing as it should.
This device solves the same problem as secondary air injection, by the opposite method. The problem is that the first minute after a cold start produces the worst emissions a car makes, because the catalyst converts almost nothing until it is hot. Secondary air injection attacks it by burning the unburnt fuel early, generating heat — that is P2257. An adsorption catalyst instead simply traps the hydrocarbons on a zeolite substrate, holds them while the exhaust is cold, and releases them back into the stream once the main catalyst is up to temperature and able to deal with them.
So it is a delay line rather than a converter, and that shapes what failure looks like. It does not stop working suddenly. The substrate loses storage capacity gradually with age, thermal cycling and contamination, so the code appears when capacity has fallen below what the strategy assumes rather than when something breaks. There is usually no symptom at all — the driver notices nothing, and the fault is found at an emissions test or by the warning light.
Before concluding the trap itself is finished, the same caution applies as for any catalyst efficiency code: the judgement is made using oxygen sensors, so a sensor reporting inaccurately produces this verdict on a healthy device. The corpus makes that argument at P2270 and P2232, and it is worth the same weight here, because this is an expensive exhaust component.
Likely causes
Start here
- Degraded hydrocarbon adsorber substrate · specialist
- Faulty oxygen sensor · moderate
Less common
- Exhaust leak before the sensor · moderate
- Excessive oil or coolant consumption · specialist
- Repeated cold starts on very short journeys · easy
Symptoms
Components involved
Information, not instruction
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.
Community experience
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