Emissions control: EGR, DPF, catalytic converter and AdBlueGeneric OBD-II codeUrgency: Garage soon
Code P0491: insufficient secondary air (bank 1)
P0491 means: secondary air injection system: insufficient flow (bank 1).[1][2][3][4] The secondary air system pumps fresh air into the exhaust, mainly with the engine cold, so that the catalytic converter reaches its working temperature sooner and emissions are reduced. The control unit monitors the system's air pressure and flow as voltage signals; if they do not match what is expected, it stores P0491.[2][5][6]
What code P0491 means
The secondary air system pumps fresh air into the exhaust, mainly with the engine cold, so that the catalytic converter reaches its working temperature sooner and emissions are reduced. The control unit monitors the system's air pressure and flow as voltage signals; if they do not match what is expected, it stores P0491.[2][5][6]
Picoauto describes the case of a Seat León II with the warning light on and P0491. The secondary air pump did not respond to the actuator test: the relay clicked, but the pump did not move. The cause was a relay with burnt contacts, and the irregular current draw of the pump also pointed to a deteriorating pump motor. The car lost no power and its driving did not change.[5]
P0492 is the same warning on bank 2. It is a generic code of the OBD-II standard (it starts with P0), with the same definition on any brand; what changes from one engine to another is the most likely cause.[7][8][9]
Original definition (SAE J2012): Secondary Air Injection System Insufficient Flow Bank 1[1][2][3][4]
Codes from the same family
They indicate the same problem in a specific cylinder, bank or sensor. Causes, symptoms and diagnosis are those on this page.
Causes of P0491 and its family
From most to least likely, according to the sources.
- 1Secondary air pump fuse or relay in poor condition.[1][2][5]
- 2Faulty secondary air pump.[2][5][6][10]
- 3Faulty system valves: combined valve, non-return valves or solenoid valves.[1][2][5][6]
- 4Hoses or pipes between the pump and the valve leaking or cracked, and vacuum leaks.[1][2][5][10]
- 5Air injection holes in the cylinder head blocked by carbon deposits.[1][2][5]
- 6Faulty secondary air pressure sensor (not all cars have one).[1][2][5][10]
- 7Damaged wiring or connectors, or loose sensor connections.[2]
- 8Software or control unit fault, according to CarWiki.[10]
Symptoms
- Engine fault warning light on.[1][2][5]Engine warning light: faults that cause it
- In many cases no change in driving or loss of power.[5]
- The air pump does not respond in the diagnostic equipment's actuator tests.[5]
- Hissing from the secondary air system.[2]
- Slow acceleration or reduced power on some cars.[2][10]Less power: faults that cause it
- The engine stalls at idle or on starting, according to PartsAvatar.[2]Engine stalls: faults that cause it
How to diagnose P0491
In order: from the most likely and cheapest to check to the most expensive.
- 1Confirm the code and note the data. Clear it, carry out a road test and scan again to see whether it is intermittent.[2]
- 2Check the secondary air pump fuse and relay.[1][2][5]
- 3Inspect the pump wiring and connectors for damage or short circuits.[2][5]
- 4Check the system hoses and fittings (cracks, heat damage, leaks) and the vacuum hoses.[2][5]
- 5Carry out the actuator test with the diagnostic equipment: if the relay clicks but the pump does not move, measure the voltage and current draw at the pump.[5]
- 6Check the non-return valves and the combined valve: air should only pass in one direction. PartsAvatar warns that replacing the pump without first checking the non-return valves can end in a new pump failure.[1][2]
- 7Check whether the injection holes in the cylinder head are blocked by carbon, and clean them.[1]
- 8If the system has a secondary air pressure sensor, check it.[1][2]
Can I keep driving with P0491?
Garage soon
AutoDTCs rates it as moderate severity: it should be fixed soon, but it normally does not leave the car stranded. In Picoauto's case the car lost no power and its behaviour did not change, and the system works mainly with the engine cold. It is worth repairing soon, but it is not a code that requires stopping immediately.[3][5]
Documented faults related to P0491
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
Catalytic converter: the most serious faults by engine and car
- Medium severityFrequency: Common1.3, 1.5 and 1.6 gasolina (serie M)Faulty catalytic converter and lambda sensors on the 2005-2010 Swift (engine warning light)On the 2005-2010 Swift the catalytic converter often fails at around 50,000 km according to gebrauchtwagenberater.de, and several 1.6 owners cite the lambda sensor. The warning is the engine light; on the Sport the catalytic converter is integrated with the manifold.
- Critical severityFrequency: Rare1.6 GDI / T-GDIPiston and connecting rod damage from catalytic converter overheating in the naturally aspirated 1.6 GDIIn the US, Kia recalled the 2012-2016 Soul with the naturally aspirated 1.6 GDI: a catalytic converter that overheats can disturb combustion and damage pistons, with a risk of breaking a connecting rod and puncturing the block. We have not found an equivalent warning for this engine in European cars.
- Low severityFrequency: Occasional1.0 VVT-iFaulty lambda sensor (P0420 code) on the 1.0 VVT-iOn the 1.0 VVT-i (1KR-FE) in the Aygo, C1 and 107/108, the lambda sensor, especially the one upstream of the catalytic converter, is documented as failing at high mileage and leaving code P0420 and the engine warning light. It is a cheap repair and does not damage the engine.
- Low severityFrequency: Rare1.6 16v HR16DE / H4MNo documented pattern of chain, catalytic converter or coil failuresUnlike other turbo engines from the same alliance, the sources consulted do not document a pattern of timing chain, catalytic converter or ignition coil failures for this naturally aspirated block: across more than 100 combined accounts, these sections appear empty or, at most, with an isolated case.
- Medium severityFrequency: OccasionalJeep Wrangler JLGearbox wiring harness with insufficient clearance from the catalytic converterTwo British campaigns from 2020 (V78 and W19) asked for the gearbox wiring harness of 664 and 547 Wranglers built between October 2017 and January 2020 to be checked, because it sat too close to the catalytic converter. It is inspected and repositioned, or replaced if the heat has damaged it.
Frequently asked questions
What is the secondary air system for?
It injects fresh air into the exhaust, mainly with the engine cold, so that the catalytic converter reaches its working temperature sooner and emissions are reduced.[5][6]
Can I keep driving with P0491?
AutoDTCs rates it as moderate severity and says it normally does not leave the car stranded. In Picoauto's case the car lost no power. It is still worth repairing soon, because the system exists to reduce emissions.[2][3][5]
Does the secondary air pump need replacing?
It is not the first step. The sources advise checking the fuse, the relay, the hoses, the non-return valves and the combined valve, and the holes in the cylinder head first. PartsAvatar warns that if the non-return valves are not checked, the new pump may end up failing.[1][2][5]
And if the code is P0492?
It is the same problem on bank 2: insufficient flow in the secondary air system. It is diagnosed in the same way, checking the valves and ducts of that bank.[7][8]
Will I pass the MOT with code P0491?
If the code has switched on the engine warning light, no: with that light on, the inspection records a serious defect, and on Euro 5 and Euro 6 cars the ITV (Spain's roadworthiness inspection, similar to the MOT) also reads the OBD diagnostic system. Repair the fault before you go; clearing the code just before the inspection does not help either, because it counts as a serious defect.[11]
Not sure what is wrong with your car? Describe what you notice in the symptom diagnosis or see all fault codes.
Sources
Every fact on this page links to its source. If you find an error, let us know.
- P0491 - Secondary Air Injection System: Bank 1: Insufficient FlowRoss-Tech Wiki · Technical documentation · consulted 31 October 2026
- Error Code P0491: What It Means & What To Do?PartsAvatar · Technical documentation · published 2 June 2022 · consulted 31 October 2026
- P0491 – Secondary Air Injection System Insufficient Flow Bank 1 — AutoDTCsAutoDTCs · Technical documentation · consulted 31 October 2026
- p0491 Generic DTC: Secondary Air Injection System Insufficient FlowDTCSearch.com (OBD Solutions) · Technical documentation · consulted 31 October 2026
- Seat Leon II | EML On and Fault Code P0491Pico Technology (PicoAuto) · Technical documentation · published 27 November 2017 · consulted 31 October 2026
- P0491 - Secondary air injection system low flow Bank one | TaTThe Automotive Technician (TaT) · Technical documentation · consulted 31 October 2026
- P0492 – Secondary Air Injection System Insufficient Flow Bank 2 — AutoDTCsAutoDTCs · Technical documentation · consulted 31 October 2026
- p0492 Generic DTC: Secondary Air Injection System Insufficient FlowDTCSearch.com (OBD Solutions) · Technical documentation · consulted 31 October 2026
- On-board diagnosticsWikipedia · Other source · consulted 31 October 2026
- OBD DTC Fehlercode P0491 | Sekundärlufteinblasung, Zylinderreihe 1 – FehlfunktionCarWiki.de · Technical documentation · consulted 31 October 2026 · in German
- Desmontando mitos: ¿puede pasar mi coche la ITV con el testigo de «avería motor» encendido?Diariomotor · Motoring press · published 8 March 2021 · consulted 2 October 2026 · in Spanish