Is it safe to drive with this code?

In many cases you may drive short distances without immediate danger, though engine performance can be rough and there is a small risk of stalling. Because the fault relates to emissions control rather than core engine function, the main concern is increased pollutant output and potential long-term damage to the catalytic converter if left unaddressed—see our general disclaimer for individual circumstances.

It may affect your MOT result if the malfunction indicator lamp is illuminated at the time of the test, as emissions-related warning lights are part of the assessment. Even without a lit lamp, underlying emissions faults can sometimes cause a vehicle to exceed permitted limits—refer to our MOT disclaimer for full details.

Roughly how much does it cost to fix?

Repair costs typically fall in the low-to-mid three-figure range, though they can reach the high three-figure range or occasionally higher depending on the root cause. A simple relay or hose replacement might be at the lower end, whilst a failed pump, valve cleaning, or carbon removal from cylinder-head passages (which may involve substantial labour) pushes costs upward. Dealer rates tend to be higher than independent specialists.

Does this code only appear in cold weather?

Technicians commonly observe that P0491 faults occur more frequently during cold spells, often because moisture or condensation can enter the pump through a defective check valve and freeze, binding the pump mechanism. However, the code can appear year-round if other components—such as clogged passages, faulty valves, or electrical faults—are the underlying issue.

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← All fault codes | Emissions

P0491

Powertrain

Secondary Air Injection System Insufficient Flow Detected Bank 1

The engine control module has detected that the secondary air injection system on Bank 1 (the side containing cylinder #1) is not supplying enough airflow into the exhaust during cold starts, which typically helps reduce emissions.

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ⓘ Information only. This page provides general educational information about fault code P0491. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.

Recommended next steps

Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.

What does P0491 mean?

P0491 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Secondary Air Injection System Insufficient Flow Detected Bank 1.

This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.

About Emissions codes

Emissions codes relate to systems that control or monitor exhaust gas composition, catalytic converters, oxygen sensors, exhaust gas recirculation (EGR), and the evaporative emissions (EVAP) system. These rarely affect drivability dramatically but can cause MOT failures.

P0491 is one of 258 codes in this category. Browse all Emissions codes →

What we know about P0491 specifically

We record the SAE J2012 standardised definition and the system this code belongs to, but we have not yet published detailed driver-reported symptoms, common causes, or step-by-step diagnostic guidance for P0491.

Rather than invent that information by generating generic content from a template, we leave it out. For specific guidance, the most reliable sources are:

•your vehicle's own service manual or manufacturer-specific technical bulletins
•a qualified mechanic with a manufacturer-level diagnostic tool
•a marque specialist familiar with your make and model

Codes nearby

Codes adjacent to P0491 in the P-code series. Many fault codes appear together with their neighbours because they relate to the same circuit or component.

P048E Exhaust Pressure Control Valve Position Sensor/Switch Circuit High

P048F Exhaust Pressure Control Valve Position Sensor/Switch Circuit Intermittent

P0490 EGR Control Circuit High

P0492 Secondary Air Injection System Insufficient Flow Detected Bank 2

P0496 EVAP (evaporative emission) Flow During A Non-Purge Condition

P0497 Evaporative Emission System Low Purge Flow

Other Emissions codes

Other fault codes in the same system as P0491. The ones with detailed guides have plain-English explanations, symptoms, causes, and diagnostic steps.

P0401

EGR Flow Insufficient Detected Detailed guide

P0402

EGR Flow Excessive Detected Detailed guide

P0403

EGR Circuit Malfunction Detailed guide

P0404

EGR Circuit Range/Performance Detailed guide

P0405

EGR Sensor A Circuit Low Detailed guide

P0420

Catalyst System Efficiency Below Threshold (Bank 1) Detailed guide

Spotted an issue or have something to add? Mechanics, technicians, and drivers who have first-hand experience with P0491 can submit a correction or expansion. We review every submission before publishing, mechanic submissions are prioritised.

Information only, not professional advice

The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →

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