OBD-II Diagnostic Trouble Code
P0440 — Evaporative Emission System Malfunction
P0440 is a general fault in the evaporative emission (EVAP) system, which traps fuel vapor from the tank in a charcoal canister and purges it into the engine. The ECM detected that the system could not hold vacuum or pressure, or did not behave as expected during its self-test. It appears on gasoline medium-duty trucks; diesel trucks do not use this EVAP system.
Severity 2/5
Less common on commercial trucks
Service soon
Severity
Service soon
Evaporative Emission System Malfunction
Code
P0440
Category
Powertrain
Subsystem
Emissions
Truck-relevant
Less common
What this means for truckers
P0440 (Evaporative Emission System Malfunction) — gas engine code. Diesels don't have EVAP systems (no fuel vapor to capture). If you see this on a diesel chassis, scanner is misreading. On gas medium-duty trucks: leaking gas cap (cheapest fix — $20), purge valve, or vapor canister.
Before you replace parts — try this first
P0440 on the Emissions system is most often loose, damaged or wrong fuel cap — 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
Typically only the check-engine light with no driveability change. Occasionally a fuel smell around the tank, difficulty filling the tank (pump shutting off early) or a rough idle right after refueling if the purge valve is stuck open.
Common causes
- CommonLoose, damaged or wrong fuel cap ($20–$60)
- CommonCracked EVAP hose or leaking connection ($100–$350)
- OccasionalFaulty purge valve or vent valve ($150–$450)
- OccasionalDamaged charcoal canister or leaking fuel filler neck ($250–$800)
How to fix
Inspect and replace the fuel cap, clear codes and drive through several cycles
When to use: First step, especially if the code appeared after refueling
Perform an EVAP smoke test to locate leaks
When to use: When the cap is good and the code returns
Command purge and vent valves with a scan tool and check operation
When to use: When no leak is found by smoke testing
Replace the canister or filler neck if smoke or damage is found there
When to use: When the leak is traced to those components
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 P0440?
P0440 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 P0440 on a commercial truck?
Total repair cost ranges $20–$800 depending on the root cause. The cheapest fix is usually the first one listed below — Loose, damaged or wrong fuel cap — 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 P0440 fail a DOT inspection?
Yes — P0440 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 P0440 and related codes?
P0440 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 P0440 is set without the companions, the issue is more isolated.
How do I prevent P0440 from coming back after a repair?
For P0440 (Evaporative Emission System Malfunction), 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.