Secondary Air Injection Relay A Circuit Malfunction
The P0418 code indicates a malfunction in the electrical circuit for relay 'A' of the secondary air injection system. This system pumps fresh air into the exhaust manifold during cold starts to reduce emissions. The relay controls power to the air pump. A circuit fault (open, short, or high resistance) prevents the relay from operating correctly, often due to a blown fuse, bad relay, wiring issues, or a faulty pump motor.
1. Most Common Causes
- Faulty secondary air injection relay
- Damaged, corroded, or loose wiring/connectors in the relay circuit
- Short to ground or voltage in the relay circuit
- Open circuit in the relay wiring
- Secondary air pump mechanical or electrical failure affecting the relay circuit
- Blown fuse for the secondary air injection circuit
- Internal ECM/PCM failure (less common)
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Failed emissions test due to increased cold-start emissions
- Possible rough idle or hesitation during cold start
- No audible air pump operation on cold start
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm P0418 and check for related codes.
Step 2: Inspect the secondary air injection relay and fuse; test for power and ground at the relay socket.
Step 3: Check wiring and connectors between the relay, PCM, and air pump for damage or corrosion.
Step 4: Test the air pump motor by applying direct battery power; if it doesn't run, replace the pump.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, but the vehicle may fail an emissions test and could have increased emissions. The check engine light will remain on.
How much does it cost to fix?▼
Typically $50-$150 for parts and labor, depending on whether the relay, fuse, or pump needs replacement.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.