P0410 (Secondary Air Injection System Malfunction): This is a general code indicating a problem somewhere in the SAI system.
P0491 (Secondary Air Injection System Pump Relay Circuit Malfunction): This code points specifically to a problem with the relay that controls the air pump.
Here's a breakdown of how to troubleshoot and potentially fix these codes:
1. Diagnosis:
* Visual Inspection: Start by visually inspecting the air pump itself. Look for any obvious damage, loose connections, or broken wiring. The pump is usually located near the engine, often under a heat shield.
* Check the Relay: Locate the secondary air injection pump relay (consult your owner's manual or a repair manual for its location in your specific Fusion model). Try swapping the relay with a known good relay of the same type. This is a quick and easy test to rule out a faulty relay.
* Check the Fuse: Verify the correct fuse for the SAI system isn't blown.
* Check Vacuum Lines: Inspect the vacuum lines associated with the SAI system for leaks, cracks, or disconnections. These lines control the operation of the air pump.
* Air Pump Operation: With a helper, you could try to hear the pump activate upon starting the engine. You might need a stethoscope to pinpoint the sound. A faulty pump will either not activate or make unusual noises.
* OBD-II Scanner: A more advanced OBD-II scanner can provide more specific data about the SAI system, possibly pointing to the exact component at fault. It might provide information on pump voltage, current draw, and other relevant parameters.
2. Common Causes and Fixes:
* Faulty Air Pump: The most common culprit. Replacement is often necessary.
* Bad Relay: Replacing the relay is a simple and inexpensive fix.
* Blown Fuse: A blown fuse is easily replaced.
* Vacuum Leak: Repair or replace any damaged vacuum lines.
* Wiring Harness Issues: Check for broken, corroded, or damaged wires in the harness leading to the air pump and relay. Repair or replace damaged wiring as needed.
* Check Valve Problems: The SAI system may incorporate check valves to prevent backflow. A faulty check valve can cause these codes.
3. Repair Procedure (General):
Replacing the air pump, relay, or vacuum lines often requires basic mechanical skills. If you're not comfortable working on your car, it's best to take it to a qualified mechanic. Repair procedures will vary slightly based on your Fusion's specific engine and configuration. Consult a repair manual specific to your vehicle's year and model for detailed instructions.
Important Considerations:
* Repair Manual: Investing in a repair manual specific to your 2008 Ford Fusion is highly recommended. It will provide detailed diagrams, specifications, and step-by-step instructions.
* Safety: Disconnect the negative battery terminal before working on any electrical components to prevent accidental shorts or injury.
* Emissions: Failure to repair the SAI system could result in a failure to pass emissions testing in your area.
In summary, diagnosing codes P0410 and P0491 requires a systematic approach. Start with the simplest checks (visual inspection, relay, fuse) and progressively move to more complex diagnostics if necessary. If you are not experienced with auto repair, seeking professional help is the safest and most efficient course of action.
