P0418
Secondary Air Injection System Relay A Circuit MalfunctionP0418 is a generic OBD-II powertrain diagnostic trouble code: Secondary Air Injection System Relay A Circuit Malfunction. It is logged by the engine control unit when the egr monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0418 means
P0418 is set when the PCM detects an abnormal voltage or current signal in the control circuit of Secondary Air Injection (SAI) relay A — the relay responsible for delivering battery power to the SAI pump motor. The SAI system exists solely to reduce cold-start emissions: immediately after a cold engine start, combustion temperatures are too low for the catalytic converter to be fully effective, so the SAI pump injects fresh ambient air directly into the exhaust manifold. This extra oxygen promotes further oxidation of hydrocarbons (HC) and carbon monoxide (CO) in the exhaust stream, dramatically reducing tailpipe emissions during the critical first few minutes of operation.
The PCM activates relay A by grounding its control coil, which closes the relay and routes battery voltage — typically through a dedicated fuse or fusible link — to the SAI pump motor. The module simultaneously monitors the circuit for the expected current draw to confirm the pump has started. If the commanded relay state and the monitored circuit voltage are inconsistent (e.g. the relay is commanded on but no current flows, or voltage is present when it should not be), the PCM sets P0418 and illuminates the MIL.
Because the SAI system operates only for the first 30–120 seconds after a cold start, the code may appear intermittently and can be difficult to reproduce on a warm engine. Cold-climate vehicles are particularly susceptible: water can enter the SAI check valve or pump housing, freeze, and mechanically jam the pump or blow the relay fuse when the pump tries to start against the ice. Wiring near the exhaust manifold is also prone to heat-induced insulation cracking and connector corrosion over time.
P0418 has no effect on normal warm-engine driving performance. Its primary consequences are a failed emissions inspection and the potential for the PCM to disable the SAI pump, leaving cold-start HC/CO emissions uncontrolled. The code does not require immediate vehicle immobilisation but should be resolved before the next emissions test or prior to operation in regions with strict cold-start emission standards.
Common causes
Most-frequently reported root causes when P0418 is logged.
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1
Defective SAI relay A (most common — relay contacts fail open or coil burns out)
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2
Blown SAI system fuse or fusible link due to pump overload or water ingress
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3
Corroded, broken, or disconnected wiring in the relay control or power circuit
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4
Damaged or corroded relay socket/connector terminals
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5
Seized or failed SAI pump motor drawing excessive current and overloading the relay
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6
Water or ice ingress into the SAI pump or check valve freezing the pump
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7
Open or high-resistance ground circuit for the relay driver
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8
Failed PCM relay-driver output (rare)
Symptoms drivers notice
How to diagnose P0418
A typical diagnostic flow when this code is present.
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1
Retrieve all stored codes and freeze-frame data with a scan tool; note if P0410 (AIR system), P0412 (switching valve A), or others are also present — multiple codes narrow the fault location.
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2
Inspect the SAI relay and its fuse: remove the relay, check fuse continuity, and test the relay coil resistance and contact operation with a DVOM; replace if outside spec.
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3
Visually inspect the wiring harness from the PCM relay-driver pin to relay socket A and from the relay output to the SAI pump for melting, chafing, or corrosion near exhaust heat sources.
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4
With the relay removed, check for battery voltage at the relay power input pin and a good chassis ground at the ground pin; verify the PCM control pin pulses to ground when the SAI activation condition is met (cold-start key-on).
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5
If the circuit is intact, test the SAI pump directly: apply battery voltage to the pump motor terminals and verify it spins freely; a pump that draws excessive current or does not spin confirms motor failure.
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6
Check the SAI check valve(s) for blockage or freeze damage — restricted valves raise pump current, blow fuses, and damage relays.
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7
Clear codes, perform a cold-start drive cycle, and confirm whether the code returns; if intermittent, use a data logger to capture relay voltage during the SAI activation window.
Related powertrain codes
- P0400 — Exhaust Gas Recirculation Flow Malfunction
- P0401 — Exhaust Gas Recirculation Flow Insufficient Detected
- P0402 — Exhaust Gas Recirculation Flow Excessive Detected
- P0403 — Exhaust Gas Recirculation Circuit Malfunction
- P0404 — Exhaust Gas Recirculation Circuit Range/Performance
- P0405 — Exhaust Gas Recirculation Sensor A Circuit Low
Frequently asked questions
Does P0418 affect engine performance or is it safe to drive?
In most cases there is no effect on warm-engine driving performance. The SAI system only operates for the first minute or two after a cold start. The main risk is failing an emissions test and higher cold-start pollutant output.
Why does this code appear more often in winter?
Water can enter the SAI check valve or pump housing and freeze overnight. When the pump tries to start against ice, it draws very high current, blows the fuse, or burns out the relay. Inspect the check valve and pump for water damage if the code appears after cold weather.
Can I clear the code and pass an emissions test?
Clearing the code resets readiness monitors, which typically causes an immediate emissions test failure regardless of the underlying fault. The SAI readiness monitor must complete a full cold-start drive cycle before the vehicle will pass. Fix the fault first.
Is the relay or the pump more commonly at fault?
The relay is more frequently the direct cause of P0418 specifically, because P0418 targets the relay control circuit. However, a dying pump that draws excessive current often takes the relay with it — replace both if the pump shows signs of overload.
Disabling P0418 in software
RaceTune can permanently disable P0418 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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