Symptoms of P0418
- Check-engine light, often with no obvious change in how the car drives.
- A failed emissions test or incomplete readiness monitors.
- Sometimes a faint fuel-vapour smell (on EVAP codes).
Common causes of P0418
- A worn catalytic converter or an oxygen/air-fuel sensor reading out of range.
- EVAP leaks — a loose or failed gas cap, cracked hose, or purge/vent valve.
- Exhaust leaks, fuel-trim problems, or a fault in a related monitor.
Useful checks before replacing parts
- Check control A relay command, pump feed voltage, fuse condition, ground path, current draw, and cold-start secondary air monitor data.
- Inspect relays, pump wiring, connectors, water intrusion, seized pump load, check valves, and harness damage near exhaust heat.
- Confirm whether control A can energize the pump or valve path and whether the monitored current matches the commanded state.
How serious is P0418, and can you keep driving?
- Urgency
- Low to moderate for driving, certain to fail a test
- Safe to drive?
- Generally yes — but the car will not pass an emissions test.
Evaporative-emission and similar codes mostly describe a system that stops fuel vapour escaping, so the car drives normally with one set. The practical cost is the test: a stored code, or monitors that have not completed, is a failure on the spot in most jurisdictions. Catalyst-efficiency codes deserve more care, because they frequently report a converter that has been damaged by something upstream — a misfire or a rich mixture — which is still there and still doing it.
Clearing P0418, and making it stay gone
Clearing P0418 with a scan tool switches the lamp off; it does not fix anything, and it throws away the freeze-frame snapshot the car saved when the fault set. That snapshot is the most useful diagnostic evidence you have, because it records the load, RPM and temperature at the moment the test failed. Read it before you erase it.
Left alone, the lamp goes out on its own once the monitor that failed runs and passes on three consecutive driving cycles. The code stays in history after that, which is how a workshop can still see it. Disconnecting the battery is not a shortcut — it clears the stored codes and every learned fuel-trim value with them, so the car relearns from scratch and the underlying fault simply sets the code again.
On vehicles from roughly 2010 onward there is also a permanent DTC, which no scan tool can erase at all. It clears only when the monitor genuinely passes. That is deliberate: it stops a code being wiped in the car park outside an emissions test.
What P0418 costs to put right
There is no honest single figure for P0418, and any page that gives you one is guessing. The code names a circuit or a failed monitor, not a part — the same code can be a chafed wire, a connector full of water, or the component itself, and those are three very different bills.
The number worth budgeting for is diagnostic time. An hour of it is almost always cheaper than the second part you buy after the first one does not fix it, and parts fitted on a guess are rarely refundable once installed. If you are getting a quote, ask what the diagnosis found rather than what is being replaced — a emissions fault traced to a specific cause is a repair, and a list of parts is a hopeful shopping list.
Questions people ask about P0418
What does P0418 mean?
P0418 indicates a malfunction in Secondary Air Injection System Control A circuit. The controller has detected an electrical problem in the control path for the A side of the air injection system.
Can I drive with P0418?
Generally yes — but the car will not pass an emissions test. Evaporative-emission and similar codes mostly describe a system that stops fuel vapour escaping, so the car drives normally with one set. The practical cost is the test: a stored code, or monitors that have not completed, is a failure on the spot in most jurisdictions. Catalyst-efficiency codes deserve more care, because they frequently report a converter that has been damaged by something upstream — a misfire or a rich mixture — which is still there and still doing it.
What causes P0418?
The usual candidates are a worn catalytic converter or an oxygen/air-fuel sensor reading out of range; eVAP leaks — a loose or failed gas cap, cracked hose, or purge/vent valve; exhaust leaks, fuel-trim problems, or a fault in a related monitor. The code tells you which test failed, not which of those did it — that is what the checks on this page are for.
Will P0418 clear itself?
Left alone, the lamp goes out on its own once the monitor that failed runs and passes on three consecutive driving cycles. The code stays in history after that, which is how a workshop can still see it. Disconnecting the battery is not a shortcut — it clears the stored codes and every learned fuel-trim value with them, so the car relearns from scratch and the underlying fault simply sets the code again.
Does P0418 mean I need to replace a part?
Do not replace the pump before checking control A relay, power feed, ground, and load current.
How much does it cost to fix P0418?
There is no honest single figure for P0418, and any page that gives you one is guessing. The code names a circuit or a failed monitor, not a part — the same code can be a chafed wire, a connector full of water, or the component itself, and those are three very different bills.
Related OBD-II codes
These nearby emissions entries can help compare wording and pin down the fault area.