What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles (Dodge, Ram, Ford, GMC, Chevrolet, VW, Toyota, etc.). Although generic, specific repair steps may vary by make/model.
When I encounter a stored P0418 code, I know the powertrain control module (PCM) has detected a malfunction in a secondary air injection (SAI) system relay circuit. The A designation indicates either the sole relay or a specific relay if multiple exist in a system.
Modern automobiles use SAI systems as a method of reducing harmful exhaust emissions by introducing ambient air into the engine’s exhaust system. SAI designs include either a belt-driven pump or an electric pump to draw ambient air, slightly compress it, and inject the pressurized air into the engine’s exhaust manifolds. The PCM electronically controls one-way valves, switching valves, anti-backfire valves, and bypass valves to regulate air pressure distribution and protect the SAI pump.
SAI system pressure fluctuations are monitored by the PCM using voltage input signals from one or more pressure sensors. When SAI pressure falls below a certain threshold, the PCM electronically opens necessary valves and allows secondary air to be pumped into the exhaust system. Engine temperature, engine RPM, and vehicle speed are also factored into SAI pump activation.
In vehicles equipped with electric pumps, the PCM monitors SAI pressure sensors and activates the pump when needed. The system relay serves to transfer battery voltage from the fuse panel or fuse link to the SAI pump.
If the PCM detects a voltage input signal from the SAI relay circuit (designated A) indicating a malfunction has occurred, a P0418 code will be stored and a malfunction indicator lamp may illuminate.
Other secondary air injection system trouble codes include P0410, P0411, P0412, P0413, P0414, P0415, P0416, P0417, P0419, P041F, P044F, P0491 and P0492.
Code Severity and Symptoms
A stored P0418 code doesn’t require immediate urgent attention. The SAI system is only associated with reducing exhaust emissions and isn’t essential to engine operation.
Symptoms of this code may include:
• Frequently no symptoms appear with a stored P0418 code
• No SAI pump activation
Causes
Possible causes for this code being set include:
• Faulty SAI pump relay
• Shorted wiring, components and/or connectors in the SAI system
• Defective SAI pump
• Blown SAI system fuses
Diagnostic and Repair Procedures
To accurately diagnose a P0418 code, a diagnostic scanner, digital volt/ohmmeter (DVOM), and reliable vehicle information source (such as All Data DIY) will be necessary.
Begin diagnosis by inspecting system wiring, connectors, and the serpentine belt driving the pump (if applicable). Repair or replace burned, corroded, or damaged circuits as needed and retest the system. Check system fuses and relays at this time if the SAI pump is electrically driven.
If your visual inspection reveals no obvious malfunction, continue by connecting the scanner to the vehicle’s diagnostic port and retrieving all stored codes and freeze frame data. Note this information as it may prove helpful later. Then clear the codes and test drive the vehicle to see if P0418 resets.
Using your vehicle information source, search for technical service bulletin (TSB) entries matching the code(s) and symptom(s) presented by the vehicle in question. Data from hundreds of thousands of repairs are used to compile TSBs, and the information they contain can be very helpful in making an accurate diagnosis.
In extremely cold weather conditions, belt-driven SAI pumps are prone to seizure from frozen condensation. To prevent condensation from reaching the SAI pump, a one-way check valve is typically incorporated in the air supply hose. These valves are known to fail and allow exhaust condensation to enter the SAI pump. Ice (frozen condensation) causes the SAI pump (belt-driven) to seize or causes a fuse to blow (electric pump).
You can use the scanner to activate the electric SAI pump. Once the pump is activated, use the DVOM to test for voltage and ground. Suspect a blown fuse or faulty power relay if no voltage is detected. Use the DVOM to check for an open circuit between the power source and the pump if all fuses and relays are in good condition. Before using the DVOM to check circuit resistance, disconnect all associated controllers.
Using the DVOM, check resistance and continuity of individual circuits. Repair or replace circuits that appear open or shorted.
Additional diagnostic notes:
• Blown fuses are typically a reaction to a short circuit rather than the source of the malfunction
• Frozen pumps may thaw before operation to prevent catastrophic pump damage