Secondary Air Injection System (Bank 2)
Your engine's secondary air injection system on the right side (Bank 2) isn't working properly, which means extra air isn't being pumped into the exhaust to reduce emissions. Think of it like a bicycle pump that's supposed to inject air into the exhaust stream but either the pump is broken, the air isn't flowing, or the valve is stuck.
The ECU monitors Bank 2's secondary air pump operation and checks for proper airflow into the exhaust manifold using oxygen sensors and pump current draw. It detects faults when the pump doesn't activate on command, current is too high/low, or oxygen sensor feedback doesn't show expected post-injection changes.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Secondary Air Pump Current Draw | 4-8 amps during activation | Below 2 amps or above 10 amps |
| O2 Sensor Response Post-Injection | Voltage change within 1-2 seconds | No response or delayed response (>3 seconds) |
Code P0491 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0491 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.