Fault code P0418 – secondary air injection system relay “A” circuit malfunction

Fault code P0418 is called “Secondary Air Injection (SAI) System Relay “A” Circuit Malfunction” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0418

Error code P0418 refers to the emission system. The AIR pump directs air into the exhaust to reduce emissions. It takes in outside air and pumps it through two one-way check valves into each exhaust outlet group.

Fault code P0418 – secondary air injection system relay

A stored code means that the transmission control module (PCM) has detected a fault in the secondary air injection (SAI) relay circuit. The letter “A” indicates either a single relay, or a specific relay if there is more than one in the system.

The purpose of the system is to force atmospheric air into the exhaust system during cold starts.

This helps to burn unburned or partially burned hydrocarbon molecules in the exhaust gas stream. Which results from incomplete combustion during cold starts when the engine is running on a highly enriched fuel-air mixture.

Secondary air systems usually consist of a high-power air pump in the form of a turbine, a relay to control the pump motor on and off. As well as a solenoid and a check valve to control the air flow. In addition, there are various tubes and ducts corresponding to the application.

In vehicles equipped with electric pumps, the PCM controls the SAI pressure sensors and turns the pump on when necessary. The system relay is used to transfer battery voltage through the fuse panel or fusible link to the SAI pump.

If the PCM detects a voltage input from the SAI relay circuit that indicates a fault has occurred. Then code P0418 will be stored and the malfunction indicator light may illuminate.

Symptoms of vehicle malfunctions

The main signal that an error P0418 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. The “Check engine” control lamp on the control panel will light up (the code will be stored as a fault).
  2. On some European vehicles the contamination control light will illuminate.
  3. The SAI pump is not activated.
  4. Sometimes there may be no symptoms despite the stored fault code.

The severity of this code is not high, but the vehicle is unlikely to pass an emissions test. Since when the error P0418 appears, the toxic emissions will increase.

Factors that can cause this error code

The error code P0418 can mean that one or more of the following problems have occurred:

  • Defective SAI secondary air pump relay.
  • Defective SAI pump.
  • Moisture in pump and motor.
  • Open or interrupted power supply to SAI system due to bad connections or damaged wiring.
  • Blown SAI system fuse.

How to fix or reset OBD-2 code P0418

Some suggested steps for troubleshooting and fix the error code P0418:

  1. Connect the OBD-II scanner to the vehicle’s diagnostic connector and read all stored data and error codes.
  2. Eliminate any other errors that may be present before proceeding to diagnose code P0418.
  3. Visually inspect the electrical wires and connectors related to the SAI system.
  4. If necessary, repair or replace any shorted, broken, damaged, or corroded components.
  5. Check the SAI secondary air pump relay.
  6. Test the secondary air pump resistance.

Diagnose and repair of problems

Code P0418 is set when there is no outside air to burn off excess hydrocarbons in the exhaust system during a cold start. This causes the voltage on the front oxygen sensor not to drop to the set level.

The diagnostic procedure requires the engine to be cold, ideally with the vehicle idling for at least 10-12 hours. After that, you must connect the diagnostic tool and start the engine.

The voltage on the front oxygen sensor should drop below 0.125 volts after about 5-10 seconds. The fault in the secondary air system will be confirmed if the voltage does not drop to this value.

If the voltage does not drop to 0.125 volts, but you can hear the air pump running, check all hoses, air lines, valves and solenoids for leaks. Also remember to check all hoses, ducts and valves for obstructions such as carbon buildup or other blockages.

If the air pump won’t turn on, check all relevant fuses, relays, wiring, and pump motor for continuity. If necessary, replace or repair failed components.

When all checks have been completed but the P0418 malfunction persists, it may be necessary to remove the exhaust manifold or cylinder head. To gain access to the system ports in order to clean the fouling.

On which vehicles does this problem occur most frequently

Fault code P0418 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Audi (A4)
  • BMW
  • Chevrolet (Blazer, TrailBlazer)
  • Dodge (RAM)
  • Ford
  • GMC
  • Hummer (H3)
  • Lexus (GX460, GX470, LX570)
  • Pontiac (Grand Prix GXP)
  • Porsche (Cayenne)
  • Skoda
  • Subaru
  • Toyota (Hilux, Land Cruiser, Prado, Tacoma, Tundra)
  • Volkswagen (Jetta, Passat)

Fault code P0418 can sometimes be found with other errors. The most common are the following: P0037, P0261, P0264, P0267, P0270, P0410, P0411, P0412, P0413, P0414, P0415, P0416, P0417, P0419, P041F, P0445, P044F, P0455, P0491, P0492, P1472, P1604.

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