P0418 – Secondary air injection (AIR) pump relay A -circuit malfunction

By Reinier (Contact Me)
Last Updated 2017-07-21
Automobile Repair Shop Owner

Trouble CodeFault LocationProbable Cause
P0418 Secondary air injection (AIR) pump relay A -circuit malfunction Wiring, AIR pump relay, ECM

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What Does Code P0418 Mean?

OBD II fault code P0418 is a generic code that is defined as “Secondary air injection pump relay A -circuit malfunction”, and is set when the PCM (Powertrain Control Module) detects a failure or malfunction in the SAI (Secondary Air Injection) circuit that is associated with the relay labelled “A”. Note that some applications use more than one relay in the SAI system, and that this code refers specifically to relay “A” and its associated circuit. Refer to the manual for the application to determine which relay is labelled “A” on SAI systems that use multiple relays.

Most modern applications use a dedicated air pump to slightly compress ambient air, before blowing it into the exhaust manifold(s) to aid in oxidizing harmful exhaust emissions. Typically, SAI systems consist of an air pump that can be either belt driven or electrically operated, pressure sensors, various hoses, one-way check valves, switching valves, relays, fuses, bypass valves, and anti-backfire valves  that are all controlled by the PCM.

In practice, the PCM monitors the system pressure via inputs from one or more pressure sensors. When the PCM detects that the SAI system pressure is approaching or falls below a level set by the manufacturer, it will activate the SAI pump (in the case of electrically operated pumps) and open/switch the appropriate valves to allow compressed ambient air to be pumped into the exhaust manifolds. Note however that requirements based on engine temperature, engine speed, as well as vehicle speed have to be met before the PCM will activate the SAI system.  To reduce loads on the electrical system, the PCM feeds power to the SAI pump via a relay (relay “A” in this case) that delivers battery voltage through a fuse or fusible link to the SAI pump motor.

The image below shows a simplified schematic drawing of a typical secondary air injection system. Note the position of relay “A” relative to other components in the system.

Secondary Air Injection Schematic

What are the common causes of code P0418 ?

Some common causes of code P0418 could include the following-

  • Defective relay
  • Damaged, shorted, disconnected, or corroded wiring and/or connectors
  • Defective AIS pump/motor
  • Blown fuses and/or fusible links
  • Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any control module is replaced

What are the symptoms of code P0418 ?

Some common symptoms of code P0418 could include the following-

  • Stored trouble code and possibly an illuminated warning light
  • Vehicle may not pass an emissions test if code P0418 is present
  • In many cases, there may be no discernible symptoms other than a stored code, since SAI system activation (or otherwise) is not essential for efficient engine operation

How do you troubleshoot code P0418 ?

NOTE: Since water ingress is a common problem on SAI pumps, check that the pump is not frozen solid if this code appears in areas that have harsh winter climates. If it is suspected that the pump may be frozen, allow the pump to thaw before commencing a diagnostic/repair procedure for this code to prevent possible further damage to the application’s electrical system and/or SAI system components.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

Step 2

Identify the relevant wiring harness, and perform a thorough visual inspection. Look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors.  Make repairs as required if damage is found.

NOTE: Include fuses and/or fusible links in this inspection, but bear in mind that blown fuses are often the result of short circuits, and not the reason for the malfunction. Therefore, do NOT replace any fuses or fusible links until it is certain that the related wiring is free of short circuits.

Step 3

If no visible damage to wiring is found, locate relay “A” and test its operation as per the instructions in the manual. Pay particular attention to its internal resistance, because this value is a reasonably good indicator of its overall condition. Replace the relay with an OEM replacement if any tested value does not agree with the values stated in the manual.

Step 4

If relay “A” checks out, prepare to perform resistance, continuity, and ground integrity checks on all associated wiring, but be sure to disconnect the SAI wiring from the PCM to prevent damaging the controller during resistance tests.

Compare all obtained readings with the values stated in the manual, and make repairs or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.

NOTE: Pay particular attention to the SAI pump’s internal resistance, and replace the pump if this value does not agree with the specified value very closely. At this point, it is good idea to apply current to the pump directly (as per the instructions in the manual) from the battery with an ampere gauge incorporated into the jump wires to measure the pump’s current draw. Compare this value with the value stated in the manual, and replace the pump if its observed current draw exceeds the stated value by more than a few percent. Note that if there are no short circuits in the circuit, an excessive current draw indicates an incipient failure of the pump motor.

Step 5

The steps outlined above will resolve code P0418 in nine out of every ten instances. However, if the fault persists, suspect an intermittent fault or a defective PCM, but bear in mind that an intermittent fault is far more likely than a failed control module. Therefore, repeat all electrical tests until the fault is found and repaired, or refer the application to the dealer or other competent repair facility for professional diagnosis and repair.

Codes Related to P0418

There are no known codes that are directly related to P0418 – “Secondary air injection pump relay “A” -circuit malfunction”.

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