P0418
AIR System Control A Circuit.
- Generic
- Powertrain
- Generic code
- Low severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 4
- AreaAuxiliary emission controls
- 18
- FaultThe specific fault in that area
What it means
P0418 is a generic code for a fault in the control circuit of the secondary air injection (AIR) system, specifically the 'A' control circuit, which usually drives the air pump relay or pump control. The engine control unit sets it when the voltage or current it sees on that circuit does not match what it commanded, for example an open circuit, a short to earth or a short to supply. The exact layout, such as relay-driven pump, control module or valve, depends on the make and engine.
Symptoms
- Engine warning light on
- Secondary air pump not running at cold start, or running when it should not
- Unusual whining or no whirring from the pump for a few seconds after a cold start
- Failed emissions test or readiness monitors not completing
- Usually no change in driveability
- Possible additional secondary air system codes stored
Possible causes
- Faulty secondary air pump relay (stuck, burnt contacts or failed coil)likely
- Blown fuse in the air pump or relay supply circuitlikely
- Wiring damage, chafing, corrosion or poor connections in the control circuit between the relay and the control unitlikely
- Water or corrosion in the relay box, pump connector or pump motor, causing a short or high resistancepossible
- Failed air pump motor drawing excessive current or shorting internallypossible
- Faulty control module driver for the air system circuit, or a software fault where one is a known issue for that modelrare
How to diagnose
- Read all stored codes and freeze frame data, noting engine temperature and conditions when the fault set, and check for related secondary air codes
- Check the secondary air system fuse and relay for condition, and look for water ingress or corrosion in the relay box
- Inspect the wiring and connectors from the relay and pump back to the control unit for chafing, melting, corrosion or damage
- Listen for the pump running on a cold start and use a scan tool's actuator test, if the vehicle supports it, to command the pump or relay on and off
- Test the relay on the bench or by substitution with a known good one of the same specification, following the manufacturer's data
- With the circuit commanded, measure supply voltage, earth and control signal at the relay and pump connectors, and check wiring continuity and insulation to earth and supply
- Check the pump's current draw and that it turns freely, and inspect the air hoses and check valves for blockage or water
- Consult model-specific wiring diagrams and technical bulletins before condemning the control unit
How to fix
- Replace the faulty secondary air pump relay with one of the correct specification
- Replace the blown fuse only after finding and correcting the reason it failed
- Repair or replace damaged wiring, terminals and connectors, and seal against water ingress
- Replace the secondary air pump if it is seized, shorted or drawing excess current, and clear any water from the air lines and valves
- Where a control unit fault is confirmed, repair or replace the unit, applying a manufacturer software update only if one is a known fix for the model
Can I keep driving?
The car can normally be driven as the secondary air system only works briefly after a cold start and engine damage is unlikely. Book it in soon, as it will probably fail an emissions test and a shorted circuit could keep blowing fuses.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.