Diagnostic article Generic OBD-II
P0491: bank 1 secondary-air flow can fail at several points
Ross-Tech's P0491 guidance includes pump supply, hoses, valves, pressure feedback and blocked cylinder-head passages, with engine-specific bulletin notes.
Read before applying this article
Documented application
Ross-Tech independent VAG diagnostic guidance; match complete fault description and controller. Vehicle-specific notes do not establish universal applicability.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P0491 identifies insufficient secondary-air flow on bank 1 in Ross-Tech's reference. Its cause list covers pump fuse or relay faults, leaking pipes, solenoids, pressure-sensor problems, combination valves and carbon-obstructed cylinder-head passages. The code therefore describes an unsuccessful system result rather than identifying a failed pump.
Distinguish supply, routing and passage restrictions
An electrically unavailable pump, air lost through a hose and air blocked at the head are different diagnostic branches. As editorial reasoning, hearing the pump run would not by itself exclude the latter two. Conversely, a flow complaint does not prove the head passages need cleaning.
Bulletin applicability changes the investigation
Ross-Tech cites a bulletin for many North American Audi 3.0- and 3.2-litre applications when P0491 and P0492 occur together. It separately points to guidance for specified 2013–2015 Volkswagen 1.8T engine codes when P0491 or P2440 is present. These are distinct conditions, not one repair for every model.
Retain the complete code pattern and confirm the engine before applying either note. This independent reference supports checking the listed system branches but provides no universal airflow target, pressure specification or cylinder-head cleaning method. A repair should follow evidence identifying which part of the air path failed.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
AIR System Insufficient Flow Bank 1
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT