Fixing a P0491 code requires a systematic approach:
1. Diagnose the Specific Problem:
* Check the Secondary Air Injection Pump Relay: Locate the relay (consult your owner's manual or a repair manual for its location). Try swapping it with a known good relay of the same amperage. If the problem goes away, replace the faulty relay.
* Inspect the Secondary Air Injection Pump: Locate the pump (often near the exhaust manifold). Look for any obvious damage, loose connections, or signs of burning. Check the wiring harness leading to the pump for any breaks, chafing, or corrosion.
* Check the Vacuum Lines: The system often uses vacuum lines to control air flow. Inspect these lines for cracks, leaks, or disconnections. Repair or replace as needed.
* Check the Secondary Air Injection System Solenoid Valve: This valve controls the flow of air to the pump. Check for proper operation and electrical continuity. A stuck-open or closed valve can cause this code.
* Check the Pressure Sensor (if equipped): Some systems use a pressure sensor to monitor the air flow. A faulty sensor can trigger the code.
* Inspect the Air Injection System Check Valve: This prevents backflow of exhaust gases. A faulty check valve can lead to reduced airflow and set the code.
2. Perform Tests (Advanced):
* Use a Scan Tool: A more advanced scan tool can provide more detailed information about the system's operation, helping you pinpoint the faulty component. This might reveal which part is failing to reach the required pressure or flow.
* Pressure Testing: If you suspect a problem with the pump or other components related to airflow, you may need to perform a pressure test to determine if the system is generating the expected pressure.
3. Repair/Replace the Faulty Component:
Once you've identified the faulty part (relay, pump, valve, etc.), replace it with a new, OEM-specified part. Avoid aftermarket parts unless they have a good reputation for reliability.
4. Clear the Code and Retest:
After making repairs, clear the diagnostic trouble code (DTC) using a scan tool. Then, start the vehicle and run it for a while to see if the code returns. If it does, you might need to re-examine your troubleshooting steps.
Important Notes:
* Safety First: Work on the vehicle only in a well-ventilated area. The exhaust system is hot, so be careful not to burn yourself. Always disconnect the negative battery terminal before starting any work.
* Consult a Repair Manual: A repair manual specific to your 2007 Ford Fusion is invaluable. It will show you the location of components, wiring diagrams, and testing procedures.
* Professional Help: If you are not comfortable performing these repairs yourself, take your vehicle to a qualified mechanic. Incorrect repairs could lead to further damage.
The P0491 code itself isn't enough to determine the exact cause. Careful and systematic diagnosis is necessary to pinpoint the faulty component and fix the problem effectively.
