P0418 — Secondary Air Injection System Relay "A" Circuit Malfunction

powertrain (engine/transmission)

Is it safe to drive?

Limited use while repair is scheduled P0418 usually does not create an immediate drivability hazard, but it can reduce secondary-air operation and emissions compliance. Short-distance driving is generally acceptable if the engine runs smoothly; stop driving if severe misfire, stalling, strong fuel odor, or additional warning indications are present.

Typical symptoms

The check-engine light illuminates with P0418 stored in the powertrain controller
P0418 may remain pending or appear intermittently during relay activation
Vehicle operation may otherwise feel normal because the relay fault can affect emissions more than drivability
Cold-start hesitation, rough idle, or unstable idle may occur if commanded secondary air is not delivered
Higher exhaust emissions or an emissions-test failure may result from reduced secondary-air operation

Common causes

A failed or internally open secondary air injection system relay A
An open circuit or high resistance in the PCM-to-relay control wire
A short to power or ground in the relay A control circuit
A blown fuse, loose connector, or corroded terminal affecting relay A
A defective PCM driver or improperly operating relay A

How to diagnose with an OBD scanner

Confirm the fault with an OBD2 scanner Connect an OBD2 scanner such as ELM327 or AIScanAuto, read stored and pending codes, and save the freeze-frame data. Confirm that P0418 is active and note any related secondary-air codes before changing parts.
Inspect relay A, fuses, and connectors Locate secondary air injection relay A using the vehicle wiring information. Check the relay socket, fuse, connector terminals, and wiring for damage, heat, corrosion, or looseness. A visually acceptable relay can still fail electrically.
Test relay A operation Remove relay A and test its coil resistance and internal contacts against the service specification. Verify the exact relay part number before replacing it, and confirm that the relay is commanded on and off under the specified engine conditions.
Check the control-circuit wiring Back-probe the PCM command wire and relay coil terminal while relay A is commanded. Check for the specified control voltage, continuity, shorts to power or ground, and excessive resistance in connectors or sections of harness.
Repair and verify the P0418 condition Repair the failed relay, fuse, connector, or damaged wiring, or address a confirmed PCM driver fault. Clear the codes with the scanner, complete applicable warm and cold operating cycles, and verify that P0418 does not return.

Approximate repair cost

Independent-shop repair estimates for P0418 are typically $200–$900 USD, depending on whether relay A, the fuse and connectors, or the wiring harness requires service. Figures are estimates and vary by region.

Common questions

Does P0418 mean the secondary-air pump itself failed?

No. P0418 identifies an electrical malfunction involving relay A; pump, valve, or airflow failures normally produce different OBD2 codes and require separate testing.

Can I continue driving with P0418 stored?

Usually, yes for limited driving if the engine otherwise runs normally. Because P0418 can affect emissions and possibly cold-start operation, diagnosis should be scheduled promptly.

What should be checked first when P0418 is present?

Start by confirming the code and freeze-frame data with an OBD2 scanner, then inspect relay A, its fuse, connector, and control-circuit wiring before ordering parts.

Will clearing P0418 repair the underlying problem?

No. Clearing the code only resets the emission monitors; P0418 will return if relay A or its control circuit remains faulty when the relay is commanded.

The engine control unit runs constant checks on every sensor and actuator connected to the powertrain. When one of those checks fails — meaning the electrical signal doesn't match what the computer expects to see — it logs a diagnostic trouble code. Code P0418 falls into that category, specifically targeting the Secondary Air Injection System Relay "A" Circuit. The computer stores this code when its self-test on that particular circuit either returns an out-of-range value or cannot complete properly. Because the fault could live in the wiring, the relay itself, or even the software running the test, diagnosis requires a methodical approach before any parts are swapped out.

The secondary air injection system itself exists to help the catalytic converters warm up faster after a cold start. It pumps fresh air into the exhaust stream, which burns leftover fuel and raises exhaust temperatures quickly. The relay "A" controls power to the air pump and any associated valves. When the ECU detects that relay "A" isn't responding correctly during its monitoring routine, P0418 gets set and the check engine light illuminates. The stored freeze frame data captures engine speed, load, temperature, and vehicle speed at the exact moment the failure occurred, giving you a snapshot of operating conditions to replicate during testing.

Most vehicles continue driving normally after P0418 sets, though the secondary air injection system may be disabled and performance could suffer slightly depending on the underlying issue. The car won't typically enter a full limp-home mode unless the fault involves the main engine controller. Still, the problem shouldn't be ignored indefinitely — an unresolved issue in this circuit can lead to higher emissions, reduced fuel economy, and potential damage to downstream components if raw fuel reaches the exhaust without proper oxidation.

Common Symptoms of P0418

The check engine light remains solidly on or may flash intermittently depending on driving conditions
The secondary air injection system fails to activate during warm-up or operates inconsistently
Fuel consumption rises noticeably because unburned fuel exits through the exhaust without proper treatment
Engine performance may feel slightly degraded, particularly during cold starts
Some vehicles display reduced power modes or limp-home restrictions if the ECU interprets the fault as safety-related
Increased exhaust odors may become apparent, especially when the air pump isn't supplementing combustion

What Causes P0418?

The most frequent culprit is a failed secondary air injection system relay itself — the internal contacts wear out or the electromagnetic coil opens internally. Wiring problems rank second, including chafed insulation, corroded terminals at the connector, or loose pins that create intermittent contact. A poor ground connection at the relay mounting point can also trick the self-test into failing. Control module issues — either a bad output driver or corrupted calibration software — account for a smaller percentage of cases. Blown fuses feeding the relay circuit trigger the same failure mode, as does a short-to-ground or open circuit anywhere in the control wiring. Less commonly, a faulty pressure sensor or airflow valve within the secondary air system can generate a secondary fault that cascades into a relay code.

How to Diagnose P0418

Pinpointing the exact cause of P0418 requires systematic electrical verification before replacing any components.

Connect an OBD2 scanner and confirm P0418 is the only active code. Record the freeze frame data — this log shows engine rpm, calculated load, coolant temperature, and vehicle speed when the failure first occurred. Replicate those exact conditions during your test drive to trigger the fault reliably.
Check for any related trouble codes stored in other modules. Codes in the same circuit family often point toward the root cause. Research any applicable technical service bulletins for your specific make, model, and model year — manufacturers release software updates and known-fix campaigns that may apply to your vehicle.
Perform a thorough visual inspection of the wiring harness and connectors serving the secondary air injection relay. Look for damaged wire insulation, green or white corrosion growth on terminals, and any signs of the connector being pulled loose from vibration or engine movement.
If your scan tool includes bidirectional controls, run the actuator test for the secondary air injection relay. The pump should activate audibly and the relay should click. No response confirms a faulty relay or a power supply problem.
With the connector unplugged, measure voltage at the relay power feed and verify good ground continuity at the designated ground point. Check each control circuit wire for continuity — resistance should be under 1 ohm and there should be no shorts to battery voltage or ground where none should exist.
Perform a wiggle test on the harness while monitoring live data. Any dropout or erratic signal during physical manipulation points directly to a broken wire or loose terminal. Clear the code afterward, complete a drive cycle matching the freeze frame conditions, and verify P0418 does not return.

Repair Options and Costs

Remediating P0418 typically involves one of three repair paths depending on what testing reveals.

Wiring and connector repairs address frayed wires, corroded pins, or loose grounds. Professional labor typically runs $80 to $250 depending on accessibility and the extent of the damage.

Control module diagnosis and repair — whether a software reflash, module replacement, or driver replacement — generally costs $300 to $1,000 at independent shop rates.

Replacing a failed sensor, solenoid, or the air injection relay itself usually falls between $150 and $500 for parts and associated labor.

How Much Does Fixing P0418 Cost?

Overall repair costs for P0418 span a wide range because the underlying cause varies significantly between vehicles. The most economical fix — correcting a wiring issue or loose connector — typically totals $80 to $250. Replacing the relay or an individual sensor runs $150 to $500. At the high end, addressing a faulty engine control module or its programming can reach $1,000. A proper diagnosis before purchasing parts is the single most effective way to avoid overspending. The most expensive scenario is rarely where you should start looking.

Can You Drive With P0418?

Most vehicles remain fully drivable when P0418 is stored. The engine runs, the transmission shifts, and the car steers and stops normally. The secondary air injection system may be non-functional, which increases emissions and slightly worsens fuel economy, but neither poses an immediate safety concern. However, if the underlying fault causes intermittent stalling, prevents a cold start, or triggers warnings in the braking or steering systems, treat the situation as urgent and have the vehicle assessed immediately.