Powertrain, generic · Auxiliary emission controls

P0418 AIR System Control A Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

After a cold start the secondary air (AIR) pump blows fresh air behind the exhaust valves so that unburnt fuel burns off and the catalyst heats up faster; a shut-off valve opens the path and stops exhaust from flowing back. Control A is the drive circuit of pump A – in practice the coil of the pump relay, which the module switches to ground; the old wording was Relay A. GM uses P0418 for an open circuit (200 kΩ or more, 20 of 25 samples, MIL after two trips) and P2257/P2258 for shorts; Saab and Toyota describe the same circuit. If the pump does not run, P0411 may follow. A relay with welded contacts keeps the pump running – Subaru recalled cars because the pump could overheat.

Likely causes

Pump relay faulty (coil open, contacts burnt or welded)
Control wire or connector to the relay open or corroded
Fuse blown, often due to a seized pump
Pump stiff or blocked by water or debris
Driver in the engine module

Symptoms

  • Check engine light on
  • Pump does not run after a cold start – or runs constantly if the relay is welded
  • Secondary air readiness not set (emission test)
  • Hot pump or burning smell if it runs constantly

Repair estimate

DifficultyEasyDIY possible
Parts and labour€20–900
Time0.2–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

air pump relaysecondary air pumprelay wiringengine control module

Related codes

P0419P2257P2258P0411P2444

References: gsi.ext.gm.com, saabwisonline.com, static.nhtsa.gov, static.nhtsa.gov.