P0418 Code: Secondary Air Injection Pump Relay A Circuit
The ECU sees a fault in the relay circuit that switches the secondary air pump on and off.
At a glance
Severity: Low
- Meaning
- Secondary Air Injection Pump Relay A Circuit
- System
- Emission Controls · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $20–$1,000
- DIY difficulty
- Easy
The secondary air pump draws too much current for the ECU to switch directly, so the ECU energizes the coil of a relay and the relay's heavy contacts feed the pump. P0418 means the ECU found a problem in that relay control circuit: the coil is open or shorted, the wiring is damaged, or on some systems the voltage it sees on the pump side doesn't match what it commanded. Relays in this job lead a hard life because the pump is a big load and often seizes from moisture, which burns or welds the relay contacts. The relay is also the cheapest part in the system, which is why testing or swapping it comes first.
Can I keep driving? Safe to drive in general. If you hear the air pump humming constantly, even with a warm engine, the relay may be stuck on: disconnect the pump or pull its fuse until it's fixed, or the pump will overheat and on some cars the battery can run down.
Try this first – 5-minute fix
- Check the air pump fuse Pull the fuse marked for the air pump or secondary air system and look at the element. If it's blown, replace it with the same rating, but suspect a seized pump if it blows again.
- Swap the relay Swap the air pump relay with an identical one from another circuit, like the horn. If the pump starts working and the code doesn't return, replace the relay.
- Clear the code and do a cold start Clear the code, let the car cool completely and start it with the hood open. You should hear the pump run briefly and then stop.
How to read P0418
- P Powertrain
- 0 Generic (SAE)
- 4 Auxiliary emission controls
- 18 Specific fault
Symptoms
- Check engine light on
- Air pump never runs at cold start
- Or the pump keeps running long after starting, sometimes even with the engine warm
- Pump fuse blown
- Battery drained overnight on vehicles where a stuck relay keeps the pump powered
- P0410 stored alongside
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Failed air pump relay: open coil, burned contacts or contacts welded shut
- Water or corrosion in the relay socket or fuse box
- Blown fuse in the relay coil or pump feed
- Seized air pump overloading and damaging the relay
- Open or shorted wire between the ECU and the relay coil
- ECU relay driver fault
Diagnosis step by step
Find the relay and fuse
Use the fuse box lid or owner's manual to locate the air pump relay and fuse. On many trucks and SUVs they sit in the underhood fuse block; on some European cars they're in a relay carrier or a separate electronics box in the engine bay.
Tools: Flashlight
Swap in an identical relay
Exchange the air pump relay with an identical relay from a non-critical circuit, such as the horn, and clear the code. If the code stays away and the pump runs at the next cold start, buy a new relay.
Tools: Pliers
Measure the relay coil
Measure resistance across the relay's coil terminals, usually marked 85 and 86. Most automotive relays read roughly 50 to 120 ohms; an open reading means a dead coil.
Tools: Multimeter
Check the socket circuits
With the relay removed and the key on, one coil terminal in the socket should have battery voltage. When the pump is commanded with a scan tool, the other coil terminal should be pulled to ground by the ECU; a test light across the two should light up.
Tools: Multimeter, OBD-II scanner
Check the pump isn't seized
Jumper the relay's load terminals (usually 30 and 87) with a fused lead for a few seconds. A healthy pump spins up immediately; if it hums, stalls or blows the fuse, it's seized and has probably damaged the relay, so replace both.
Tools: Multimeter, Flashlight
Where the parts are
- 1Battery
- 2Alternator
- 3Engine computer (ECM)
- 4CAN bus
- 5Transmission
- 6Wheel speed sensor
- 7High-voltage battery
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace air pump relay | Yes | $20–$120 |
| Clean or repair relay socket or fuse box terminals | No | $60–$250 |
| Repair relay control wiring | No | $100–$300 |
| Replace seized air pump and relay together | No | $400–$1,000 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Chevrolet
On GM trucks and SUVs with the AIR system, a pump seized by water often takes out the AIR pump relay or fuse in the underhood fuse block, so P0418 and P0410 appear together. Replacing only the relay gives a short-lived fix if the pump is dragging.
Volkswagen
On many VW and Audi gasoline models of the Golf IV and Passat B5 era, a secondary air pump relay with welded contacts keeps the pump running continuously until it burns out. If the pump is still humming with a warm engine, replace the relay before fitting a new pump.
BMW
A failed secondary air pump relay is a known cause of P0418 on BMW six-cylinder gasoline engines. It's inexpensive and quick to test, so check it before condemning the pump.
Field data
| Toyota | 12 | Tundra, Tacoma | |
|---|---|---|---|
| Subaru | 2 | Impreza, Wrx |
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
P0418 with the check engine light on fails OBD-based smog checks. The secondary air monitor only runs after a cold start, so plan for at least one cold start before the retest.
Frequently asked questions
Can a relay really cause a check engine light?
Yes. The ECU monitors the relay's control circuit, and on some vehicles the pump side as well. A relay with an open coil or welded contacts shows up as P0418.
Why is my air pump running all the time?
Most likely the relay contacts are welded shut, so the pump gets power regardless of what the ECU wants. Pull the fuse to stop it, replace the relay and then check whether the pump survived.
Should I replace the pump too?
Only if it's seized or noisy. Test it by feeding it power through a fused jumper: if it spins freely and quietly, a new relay alone is enough.
Sources
- SAE J2012 – Diagnostic Trouble Code Definitions
- ISO 15031-6 – Road vehicles, diagnostic trouble code definitions (EOBD)
- Manufacturer public service information (cross-checked)
- NHTSA Complaints API
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy