OBD-II code family
Secondary Air Insufficient Flow Bank Faults (2 codes)
2 related fault descriptions. Use each full code and the exact vehicle service information to identify its circuit, monitored condition, and required tests.
Codes in this family
This is the complete published member list for this guide. Each code retains its own page; shared diagnostic context does not mean that every member has the same test or repair.
What these codes mean
P0491 identifies insufficient secondary-air flow for bank 1 and P0492 the corresponding bank 2 result. Bank numbering follows the engine’s cylinder identification. It does not establish a left/right position, longer hose, hidden component or more difficult repair.
The monitor evaluates the fitted secondary-air system’s response under defined conditions. Pump command, pump operation, valve opening, air delivery and the sensing input are different checks. Hearing a pump or seeing a valve command does not prove that air reached the required exhaust path or that the measurement was valid.
Audi bulletin 2035002/4 from March 2016 gives a concrete example for its listed 2010–2012 A8 4.2FSI applications. Low vacuum supply can let the secondary-air valves close too early during catalyst heating; its procedure considers the vacuum system and combination valves. The case shows why an insufficient-flow result is not automatic pump replacement. Its particular vacuum value, timing, parts and two-start verification are not universal instructions.
Preserve the full scan and event conditions, then identify the actual bank, pump/valve arrangement, shared supplies and diagnostic priorities. A fault on one bank does not independently clear a shared component, and codes on both banks do not prove one common failure. Have the required electrical, air-path, control and input checks performed.
Use the approved equipment and isolation before reaching exhaust or moving components. Do not blow through hoses by mouth, apply direct power to an unknown device or use an improvised pressure test. Repair the demonstrated fault, complete any required setup and verify the applicable monitor. A cleared warning alone is not completed verification, and a diagnosis-service example does not price a pump, valve or exhaust-path repair.
Identify the circuit on your vehicle
Match the full DTC, reporting module, and any failure-type suffix to the service information for your exact year, make, model, engine, and equipment. An identifier alone does not establish installed hardware count, location, connector pinout, or a universal measurement.
Where the manufacturer publishes its own meaning for this code, that is the one to work from — it describes the actual component on your vehicle, which the generic definition cannot. Keep the generic description for what it is good for: it tells you which subject area the standard assigned the code to, which is useful context when the two appear to disagree. If they do disagree, follow the manufacturer.