EN

Severity: Minor Code type: Generic (all makes)

P0440 Code: Evaporative Emission System Malfunction

Catch-all EVAP code: the system failed its self-check. Start with the gas cap, then check hoses and valves.

Safety first · Consult a professional

  • Fuel vapor is flammable. No smoking or open flames; work outdoors and fix fuel leaks before driving.

At a glance

Severity: 1/5 – Minor1/5

Severity: Minor

Meaning
Evaporative Emission System Malfunction
System
Emission Controls · Powertrain
Safe to drive?
Yes, short term
Typical repair cost
$15–$800
DIY difficulty
Easy

P0440 is the oldest and least specific member of the EVAP family. Newer ECUs split EVAP faults into leak sizes (P0442, P0455, P0456) and single-component codes (P0441, P0443, P0446), but many vehicles from the first OBD-II generations, and some current ones, simply log P0440 when the whole system fails its self-check. The system has two doors: the purge valve at the engine lets stored vapor into the intake, and the vent valve at the canister lets fresh air in or seals the tank for testing. When the ECU closes the vent, opens purge and sees no vacuum build in the tank, or when a valve doesn't respond at all, P0440 is the result. In practice a loose gas cap, a hose knocked off during other work and a stuck valve cover most cases, in that order.

Can I keep driving? Drive as normal. The only urgency comes from a noticeable raw-fuel smell, which means vapor is escaping somewhere it shouldn't; get that looked at within days rather than months.

Try this first – 5-minute fix

  1. Tighten or replace the gas cap Remove the cap, clean the seal and the neck, then screw it on until it clicks. A cap with a flattened or cracked O-ring, or a universal cap that doesn't fit the neck, should simply be replaced.
  2. Look for a hose that has come off Open the hood and follow the small hose from the purge valve on the intake back toward the firewall. After air-filter, intake or spark-plug work, this line is often left disconnected or cracked at the elbow.
  3. Check the purge valve connector Press the connector on the purge valve firmly home and make sure the locking tab is engaged. A half-seated connector stops the valve working and is enough for P0440 on many vehicles.
  4. Give the ECU time to retest Clear the code or drive for a week. The EVAP self-check runs only after a cold start with a part-full tank, and the light goes out only after the test passes on consecutive trips.

How to read P0440

How to read P0440P10243404
  1. P Powertrain
  2. 0 Generic (SAE)
  3. 4 Auxiliary emission controls
  4. 40 Specific fault

Symptoms

  • Check engine light with no other change in how the car runs
  • Faint gasoline smell around the car in warm weather
  • Light appears after refueling or after work in the engine bay
  • Rough idle just after refueling if the purge valve is stuck open
  • EVAP monitor never reaches 'ready' on the scan tool

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Gas cap loose, worn or not the correct type 30%
  2. EVAP hose disconnected, split or crushed, often at the purge line near the intake 22%
  3. Purge valve stuck or not opening when commanded 15%
  4. Canister vent valve stuck open, or its fresh-air path blocked 12%
  5. Charcoal canister cracked or soaked with liquid fuel from topping off 10%
  6. Fuel tank pressure sensor reading wrong 6%
  7. Wiring or connector fault at one of the EVAP solenoids 5%

Diagnosis step by step

Diagnostic flow: P04401Read every code, not just P04402Inspect cap and filler neck3Trace the vapor lines end to end4Command the valves and watch tankpressure5Smoke-test the sealed system6Check the canister for fuel

  1. Read every code, not just P0440

    A companion code tells you which part of the system failed. P0441 or P0443 points to the purge side, P0446 or P0449 to the vent side, and P0455 or P0457 to a gross leak. P0440 on its own on an older vehicle usually means a gross-leak result.

    Tools: OBD-II scanner

  2. Inspect cap and filler neck

    Check the cap seal for cracks and the neck lip for rust or dents. Have the cap pressure-tested at a shop or parts store if it looks tired; a cap that fails the tester is the whole repair.

    Tools: Flashlight

  3. Trace the vapor lines end to end

    Follow the purge line from the intake to the canister, then the lines from the canister to the tank and filler neck. Rubber elbows near the exhaust harden and split; plastic lines crack at the quick-connectors.

    Tools: Flashlight

  4. Command the valves and watch tank pressure

    With a scan tool that supports bidirectional control, close the vent and open purge with the engine idling. Tank pressure should drop to a few inches of water vacuum within a minute. No change means a valve isn't responding or the leak is large; a drop that collapses at once means a large leak.

    Tools: OBD-II scanner

  5. Smoke-test the sealed system

    Feed low-pressure smoke into the service port or filler neck with the vent closed. Watch the filler neck, the canister and every hose joint; on this code the leak is usually big enough to appear within a minute.

    Tools: Smoke machine, Flashlight

  6. Check the canister for fuel

    Disconnect the vent hose at the canister and look for wet charcoal, liquid fuel or loose granules. A flooded canister must be replaced, and the refueling habit that caused it has to change.

    Tools: Flashlight, Gloves

Where the parts are

EVAP system: gas cap, canister, purge and vent valves – P0440123456
EVAP system: gas cap, canister, purge and vent valves
  1. 1Gas cap
  2. 2Charcoal canister
  3. 3Purge valve
  4. 4Vent valve
  5. 5Fuel pump
  6. 6Intake manifold

Repair options & costs

RepairDIY?Estimated cost
Replacement gas cap Yes $15–$45
Reconnect or replace an EVAP hose Yes $50–$250
Replace purge valve Yes $80–$250
Replace canister vent valve Yes $120–$350
Replace charcoal canister No $300–$800

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Toyota

On older Toyotas and Lexus models, P0440 often appears alongside P0441 and P0446. The charcoal canister with its integrated vacuum switching valves is the usual culprit once the cap has been ruled out.

Field data

Jeep 4 Grand Cherokee, Renegade, Wrangler
Chrysler 2 300, Pacifica
Dodge 2 Durango, Durango Srt

Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.

Emissions / smog test impact

Will fail

A stored P0440 with the check engine light on is an automatic fail on OBD-based smog checks. After a repair the EVAP monitor must complete before the test; in most states only one incomplete monitor is allowed on newer vehicles, so give it a week of mixed driving.

Frequently asked questions

Why is the code so vague?

P0440 was defined before ECUs had the resolution to say what failed. Newer vehicles use separate codes for leak size and for each valve; older ones report the overall test result and leave the detective work to you.

What's the difference between P0440 and P0455?

P0455 explicitly reports a large leak. P0440 says the system failed its check without stating why, which on many older vehicles amounts to the same thing but can also be a valve that didn't respond.

How long until the light goes out after I tighten the cap?

The ECU has to rerun the test, which needs a cold start and a fuel level between roughly a quarter and three-quarters. Expect a few days; clearing the code with a scanner turns the light off immediately but resets the EVAP monitor to not ready.

Can I pass smog with P0440 cleared but the monitor not ready?

Usually not, if more than one monitor is incomplete, and some states insist the EVAP monitor is complete. Drive the car normally for a week before your appointment rather than clearing the code the day before.

Sources