Technical Description of OBD-II Trouble Code
Malfunction in the Circuit of Secondary Air Injection System Relay A
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to OBD-II equipped vehicles (Dodge, Ram, Ford, GMC, Chevrolet, VW, Toyota, etc.). Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
When encountering a stored code P0418, it indicates that the powertrain control module (PCM) has detected a malfunction in a relay circuit of the secondary air injection (SAI) system. The A designation signifies either the only relay or a specific relay if there are multiple relays in the system.
Modern cars utilize SAI systems to reduce harmful exhaust emissions by introducing ambient air into the engine exhaust system. SAI designs incorporate either a belt-driven pump or an electric pump to draw in ambient air, slightly compress it, and inject the pressurized air into the engine exhaust manifolds. The PCM controls various components such as one-way valves, switching valves, anti-backfire valves, and bypass valves to regulate and distribute air pressure, as well as protect the SAI pump.
The PCM monitors fluctuations in SAI system pressure using voltage input signals from one or more pressure sensors. When the SAI pressure falls below a certain level, the PCM electronically opens the necessary valves to allow secondary air to be pumped into the exhaust system. SAI pump activation also takes into account factors such as engine temperature, engine RPM, and vehicle speed.
In vehicles equipped with electric pumps, the PCM monitors SAI pressure sensors and activates the pump when necessary. The system relay transfers battery voltage from the fuse panel or fusible link to the SAI pump.
If the PCM detects a voltage input signal indicating a malfunction in the SAI relay circuit (designated as A), it will store a code P0418 and may illuminate a malfunction indicator lamp.
Other trouble codes related to the secondary air injection system include P0410, P0411, P0412, P0413, P0414, P0415, P0416, P0417, P0419, P041F, P044F, P0491, and P0492.
Severity of the Code & Symptoms
There is no urgent need to address a stored code P0418. The SAI system is only associated with reducing exhaust emissions and is not essential for engine operation.
Symptoms that may be experienced with this code include:
Causes
Possible causes for this code to be set are:
Diagnostic and Repair Procedures
To accurately diagnose a code P0418, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information (such as All Data DIY).
Begin the diagnosis by inspecting the system wiring, connectors, and the serpentine belt that drives the pump (if applicable). Repair or replace any burnt, corroded, or damaged circuits as necessary, and then retest the system. If the SAI pump is driven by an electric motor, check the system fuses and relays at this time.
Next, connect the scanner to the vehicle’s diagnostic port and retrieve all stored codes and freeze frame data, if your visual inspection does not reveal any obvious malfunctions. It is important to write down this information as it may be useful later. Clear the codes and test-drive the vehicle to see if the P0418 code is reset.
Use your vehicle information source to find technical service bulletin (TSB) entries that match the code(s) and symptom(s) exhibited by the vehicle in question. TSBs contain valuable information based on numerous repairs and can assist in reaching an accurate diagnosis. In extremely cold weather conditions, belt-driven SAI pumps are prone to seizure due to frozen condensation. To prevent condensation from reaching the SAI pump, an air inlet hose usually incorporates a one-way check valve. These valves are known to fail and allow condensation from the exhaust to enter the SAI pump. The ice (frozen condensation) can cause the SAI pump to seize (in belt-driven systems) or bind and blow a fuse (in electric pump systems).
If you have an electric SAI pump, you can use the scanner to activate it. After activating the pump, use the DVOM to test for voltage and ground. If no voltage is detected, suspect a blown fuse or a faulty power supply relay. If all fuses and relays are good, use the DVOM to check for an open circuit between the power supply source and the pump. Before checking circuit resistance with the DVOM, disconnect all related controllers.
Use the DVOM to check individual circuit resistance and continuity. Repair or replace any circuits that appear to be open or shorted.
Additional diagnostic notes:
