🔍 What Does Code P0491 Mean?

The P0491 code indicates that the secondary air injection system has insufficient flow in Bank 1. This often appears on Mercedes, BMW, Audi, and VW engines during cold starts, especially when the secondary air pump or check valve fails.

The SAI system injects fresh air into the exhaust to reduce emissions on cold start. If it fails, you might experience rough idle on cold start or a P0491 Audi A4 cold start issue.

You may also see P0492 (Bank 2) alongside this code.

🚨 Common Symptoms of P0491

• Check Engine Light on (CEL illuminated)
• Rough idle or hesitation during the first minute after startup
• Decreased fuel efficiency
• Emission test failure or not-ready status
• Audible secondary air pump noise or no sound at startup
• Occasional “P0491 code won’t clear” even after replacing the air pump

⚙️ Common Causes of P0491

• Failed secondary air pump motor
• Stuck or leaking SAI check valve
• Cracked vacuum hose or solenoid failure
• Carbon buildup in cylinder head air passages
• Faulty relay or blown fuse
• Wiring issue in the air injection system

In many Mercedes and BMW models, the secondary air injection solenoid valve can clog or stick — a common cause when you’re searching “how to test secondary air injection solenoid valve”.

🧰 How to Diagnose and Fix P0491 with iCarsoft CR MAX

1. Connect the iCarsoft CR MAX scan tool and perform a full system scan.
2. Confirm P0491 and note related codes like P0410 or P0492.
3. Go to Active Test → Secondary Air Pump to check if the pump activates. This test works on Mercedes, Audi, and VW vehicles.
4. If the pump doesn’t activate, check fuses, relays, and wiring continuity.
5. If the pump runs but flow is low, inspect the SAI check valve and hoses for carbon blockage.
6. Use live data to compare “SAI Commanded ON” vs “SAI Pressure Sensor Feedback.”
7. After repair, clear codes and perform a cold start test. If “P0491 won’t clear,” recheck carbon buildup in the ports.

📖 Real Case Example

A 2017 Mercedes C250 (2.0L Turbo) showed both P0491 and P0410. Using iCarsoft CR MAX, the technician ran the “Secondary Air Pump Test” — the pump was silent. After checking relays and vacuum lines, they discovered a stuck secondary air valve. Once replaced, the engine started smoothly, and the code cleared permanently.

Similar cases are common in Audi A4 and VW Passat — where drivers often ask online “how to fix P0491 secondary air injection insufficient flow.” The root cause is almost always a blocked check valve or deteriorated vacuum line.

💡 Related Fault Codes

• P0492 – Insufficient Air Flow Bank 2
• P0410 – Secondary Air Injection System Malfunction
• P0472 – Exhaust Pressure Sensor Low Input

❓ FAQ: P0491 Diagnostic & Fix

Q1: Is P0491 serious?

Not immediately, but prolonged driving with this fault increases emissions and can trigger inspection failure.

Q2: How do I test the secondary air pump?

Using iCarsoft CR MAX, go to “Actuation Test → Air Pump Activation.” If it doesn’t run, check power supply and ground. If it runs but flow is weak, suspect blockage.

Q3: What if P0491 returns after replacing the pump?

This is common in Audi and BMW engines due to carbon-clogged air passages. Cleaning ports may be necessary if your P0491 code won’t clear.

Q4: Can I clean the SAI valve myself?

Yes. Remove the valve, spray carb cleaner, and verify it opens when vacuum is applied. Many DIYers search “how to test secondary air injection solenoid valve” — this is exactly that process.

Q5: What tools are best for diagnosing P0491?

The iCarsoft CR MAX is ideal — it performs active tests, live data comparison, and system resets for all major car brands.