Secondary Air Injection System Switching Valve B Circuit Shorted
The secondary air injection switching valve B has an electrical short circuit, preventing proper air flow into the exhaust system during cold starts. Think of it like a water valve that's stuck closed due to a broken electrical switch.
The ECU monitors the switching valve B solenoid circuit resistance and voltage response. During cold start, the ECU commands the valve open and expects a specific current draw; a shorted circuit causes excessive current or abnormal voltage drop. The ECU detects this deviation within 2-3 seconds and sets the fault code.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Solenoid Circuit Resistance | 4-8 ohms when energized | < 2 ohms or short to ground |
| Supply Voltage at Valve | 12V nominal with 0.5-1.5A draw | Excessive current (>2A) or voltage collapse |
Code P0418 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, P0418 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.