OBD-II Diagnostic Trouble Code
P0418 — Secondary Air Injection System Relay A Circuit
P0418 means the ECM detected a fault in the secondary air injection relay A control circuit, or found that the pump does not run as commanded through the relay. The relay powers the air pump during cold-start operation.
Severity 2/5
Less common on commercial trucks
Service soon
Severity
Service soon
Secondary Air Injection System Relay A Circuit
Code
P0418
Category
Powertrain
Subsystem
Emissions
Truck-relevant
Less common
What this means for truckers
P0418 (Secondary Air Injection Relay A Circuit) — gas engine code, AIR pump relay fault.
Before you replace parts — try this first
P0418 on the Emissions system is most often failed secondary air pump relay — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Check-engine light, an air pump that does not run at cold start, or a pump that keeps running after it should stop, which can burn it out. Driving is normally unaffected.
Common causes
- CommonFailed secondary air pump relay ($30–$150)
- OccasionalOpen or shorted relay control wiring ($80–$400)
- OccasionalBlown fuse feeding the relay or pump ($20–$150)
- OccasionalSeized air pump overloading the relay ($300–$1000)
- RareECM driver fault ($1000–$3000)
How to fix
Swap the relay with a known-good identical relay and retest
When to use: First step
Check the relay coil control and supply circuits
When to use: When a new relay does not fix it
Check the pump current draw and replace a seized pump
When to use: When fuses or relays keep failing
Test the ECM relay driver
When to use: After wiring and relay are cleared
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0400 — Exhaust Gas Recirculation Flow Malfunction
- → P0401 — Exhaust Gas Recirculation Flow Insufficient Detected
- → P0402 — Exhaust Gas Recirculation Flow Excessive Detected
- → P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
- → P0421 — Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Frequently Asked Questions
Can I drive with code P0418?
P0418 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix P0418 on a commercial truck?
Total repair cost ranges $20–$3000 depending on the root cause. The cheapest fix is usually the first one listed below — Failed secondary air pump relay — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0418 fail a DOT inspection?
Yes — P0418 is an emissions-related code, and DOT inspectors check emissions compliance. An active emissions code with a check engine light can result in an out-of-service violation under FMCSA Part 393. The truck typically can't be moved until the code is cleared. Repeated emissions violations escalate to CSA scores.
What's the difference between P0418 and related codes?
P0418 is closely related to: P0400 (Exhaust Gas Recirculation Flow Malfunction), P0401 (Exhaust Gas Recirculation Flow Insufficient Detected), P0402 (Exhaust Gas Recirculation Flow Excessive Detected), P0420 (Catalyst System Efficiency Below Threshold (Bank 1)). These codes share the same subsystem (emissions) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0418 is set without the companions, the issue is more isolated.
How do I prevent P0418 from coming back after a repair?
For P0418 (Secondary Air Injection System Relay A Circuit), prevention centers on the emissions system. Keep up with PM on the EGR system — clean the valve at 200K miles. Use OEM-spec air filters. Don't ignore minor codes; they often escalate into bigger emissions failures.