Failed secondary air pump (both sources; RepairPal lists air pump replacement as a common repair). Faulty check valve at the exhaust manifold (OBD-Codes.com; RepairPal lists air pump check valve replacement as a common repair). Failed secondary air pump control valve, or a faulty combination (combi) valve (both sources). Clogged, leaking or disconnected vacuum hose (both sources). Bad vacuum control switch (OBD-Codes.com). Blown fuse or bad relay for the air pump (OBD-Codes.com). Leak in the hoses or piping between the pump and the combi valve (OBD-Codes.com). Faulty secondary air pressure sensor (OBD-Codes.com). Secondary air ports in the cylinder head plugged with carbon (OBD-Codes.com). Wiring problem (RepairPal). Failed powertrain control module (PCM), less likely (RepairPal).

Replace a failed secondary air pump. Replace a stuck or faulty check valve or combi valve. Replace a cracked, hardened, leaking or disconnected vacuum hose or air pipe. Replace a bad vacuum control switch or pressure sensor. Replace a blown fuse or bad pump relay, and repair damaged wiring. Clean carbon-clogged secondary air ports in the cylinder head, which OBD-Codes.com notes may need engine parts removed. Diagnose or replace the PCM only after everything else checks out.

P0491🌫 Emissions / secondary air injectionUsually OK to drive

P0491: Secondary Air Injection Insufficient Flow Bank 1 (Meaning, Causes, Fixes)

Q: What does P0491 mean?

P0491 means the engine computer saw too little fresh air being pumped into the exhaust by the secondary air injection system during a cold start. The usual suspects are a failed air pump, a stuck check valve or control valve, a leaking or disconnected vacuum hose, a blown fuse or bad relay, or carbon-clogged air ports in the cylinder head. The secondary air injection system pumps outside air into the exhaust for about the first minute after a cold start. RepairPal explains that the extra air helps burn off the very rich exhaust an engine produces while it warms up, cutting hydrocarbon emissions. The system typically has an electric air pump, tubes to route the air, a solenoid or vacuum control switch, and one or more check valves. OBD-Codes.com notes it is most common on Audi, BMW, Porsche and VW vehicles but can be found on others. When the powertrain control module (PCM) detects that the system is not delivering enough air, it sets P0491. It is a generic OBD-II code. The sources word it slightly differently: OBD-Codes.com names it Secondary Air Injection System Insufficient Flow Bank 1, with P0492 as the Bank 2 version (bank 1 is the side of the engine with cylinder 1), while RepairPal lists both P0491 and P0492 simply as Secondary Air Injection System Insufficient Flow without naming a bank.

Q: Can I keep driving with P0491?

Usually OK to drive. Usually safe to drive in the short term, but get it fixed. RepairPal rates P0491 Moderate, meaning it may not threaten the vehicle's safety or function right away but should be diagnosed within a reasonable time. OBD-Codes.com gives no severity rating and lists only the check engine light as a symptom. Because the secondary air system is an emissions system, a lit check engine light from this code can make the car fail an emissions inspection.

Reviewed 2026-10-23 · Based on 2 cited source(s)

What P0491 means

The secondary air injection system pumps outside air into the exhaust for about the first minute after a cold start. RepairPal explains that the extra air helps burn off the very rich exhaust an engine produces while it warms up, cutting hydrocarbon emissions. The system typically has an electric air pump, tubes to route the air, a solenoid or vacuum control switch, and one or more check valves. OBD-Codes.com notes it is most common on Audi, BMW, Porsche and VW vehicles but can be found on others. When the powertrain control module (PCM) detects that the system is not delivering enough air, it sets P0491. It is a generic OBD-II code. The sources word it slightly differently: OBD-Codes.com names it Secondary Air Injection System Insufficient Flow Bank 1, with P0492 as the Bank 2 version (bank 1 is the side of the engine with cylinder 1), while RepairPal lists both P0491 and P0492 simply as Secondary Air Injection System Insufficient Flow without naming a bank.

Severity: can I keep driving?

Usually OK to drive

Usually safe to drive in the short term, but get it fixed. RepairPal rates P0491 Moderate, meaning it may not threaten the vehicle's safety or function right away but should be diagnosed within a reasonable time. OBD-Codes.com gives no severity rating and lists only the check engine light as a symptom. Because the secondary air system is an emissions system, a lit check engine light from this code can make the car fail an emissions inspection.

Common symptoms

Check engine light on (both sources; OBD-Codes.com lists it as the only symptom)
Usually no noticeable change in how the car drives, since the system only runs for about a minute after a cold start
A failed emissions inspection while the light is on

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Failed secondary air pump (both sources; RepairPal lists air pump replacement as a common repair)
Faulty check valve at the exhaust manifold (OBD-Codes.com; RepairPal lists air pump check valve replacement as a common repair)
Failed secondary air pump control valve, or a faulty combination (combi) valve (both sources)
Clogged, leaking or disconnected vacuum hose (both sources)
Bad vacuum control switch (OBD-Codes.com)
Blown fuse or bad relay for the air pump (OBD-Codes.com)
Leak in the hoses or piping between the pump and the combi valve (OBD-Codes.com)
Faulty secondary air pressure sensor (OBD-Codes.com)
Secondary air ports in the cylinder head plugged with carbon (OBD-Codes.com)
Wiring problem (RepairPal)
Failed powertrain control module (PCM), less likely (RepairPal)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Read all stored codes and write them down; other secondary air codes such as P0410 or P0492 can point to the same problem.
Do your checks with the engine completely cold. OBD-Codes.com notes the system only runs on a cold start, so some tests may have to wait a few hours for the engine to cool again.
Check the fuse and relay for the secondary air pump and replace any that are blown or bad.
Look over the vacuum hoses and air tubes for cracks, hardened spots, loose fittings or lines that have come off. OBD-Codes.com warns many of these hoses sit under or behind the intake manifold, so some are hard to reach.
On a cold start, with a helper, listen for the air pump running and check that air comes out of its outlet hose. OBD-Codes.com then describes jumping 12 volts to the pump to see whether the pump itself or its power supply is at fault.
Clear the codes after a repair. Expect the monitor to need a cold start before it can rerun.

Typical fixes

Replace a failed secondary air pump
Replace a stuck or faulty check valve or combi valve
Replace a cracked, hardened, leaking or disconnected vacuum hose or air pipe
Replace a bad vacuum control switch or pressure sensor
Replace a blown fuse or bad pump relay, and repair damaged wiring
Clean carbon-clogged secondary air ports in the cylinder head, which OBD-Codes.com notes may need engine parts removed
Diagnose or replace the PCM only after everything else checks out

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

Testing the check valve and vacuum control switch needs a hand vacuum pump, a vacuum gauge and a multimeter, as OBD-Codes.com describes
Vacuum at the check valve control hose stays low (OBD-Codes.com expects at least 10 to 15 inches) and further testing means removing engine parts
The air ports in the cylinder head may be clogged with carbon, which is a bigger job
You have an emissions inspection coming up and need the secondary air monitor to complete

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is P0491 serious?

Not usually right away. RepairPal rates it Moderate, so it may not affect safety immediately but should be diagnosed within a reasonable time. It is an emissions fault, so it can cause a failed inspection while the light is on.

What is the difference between P0491 and P0492?

OBD-Codes.com says P0491 is insufficient secondary air flow on bank 1, the side of the engine with cylinder 1, and P0492 is the same fault on bank 2. RepairPal lists both with the same name and no bank.

What is the most common fix for P0491?

RepairPal lists air pump replacement and air pump check valve replacement as the common repairs. OBD-Codes.com also suggests checking the fuse, relay and vacuum hoses first, since those are cheaper.

How much does it cost to fix P0491?

Neither source gives a repair cost. The price depends on whether a fuse, a hose, a valve, the pump or clogged cylinder head ports are at fault, so get a diagnosis or a written estimate before replacing parts.

Sources

Last reviewed 2026-10-23. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.