OBD-II Diagnostic Trouble Code
P0442 — Evaporative Emission System Leak Detected (small leak)
P0442 indicates the EVAP system leak test found a small leak, typically equivalent to an opening around 0.040 inch. The ECM sealed the system with the vent valve, applied vacuum or pressure, and saw it decay faster than allowed. Applies to gasoline trucks with an EVAP system.
Severity 2/5
Less common on commercial trucks
Service soon
Severity
Service soon
Evaporative Emission System Leak Detected (small leak)
Code
P0442
Category
Powertrain
Subsystem
Emissions
Truck-relevant
Less common
What this means for truckers
P0442 (EVAP Small Leak) — gas engine. Most common cause: loose gas cap. Tighten cap, drive 50+ miles, code usually clears on its own.
Before you replace parts — try this first
P0442 on the Emissions system is most often fuel cap seal worn or cap not tightened — 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
Check-engine light only in most cases. A faint fuel smell near the tank or filler may be present. Performance and fuel economy are normally unaffected.
Common causes
- CommonFuel cap seal worn or cap not tightened ($20–$60)
- CommonSmall crack in an EVAP hose or loose clamp ($100–$300)
- OccasionalLeaking purge or vent valve seal ($150–$450)
- OccasionalLeaking fuel pump/sender gasket or filler neck ($250–$900)
How to fix
Replace or reseat the fuel cap and clear the code
When to use: First step on any small leak code
Smoke test the EVAP system with the vent valve closed
When to use: When the code returns after a cap replacement
Replace the leaking hose, valve or seal identified by the smoke test
When to use: Once the leak location is found
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 P0442?
P0442 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 P0442 on a commercial truck?
Total repair cost ranges $20–$900 depending on the root cause. The cheapest fix is usually the first one listed below — Fuel cap seal worn or cap not tightened — 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 P0442 fail a DOT inspection?
Yes — P0442 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 P0442 and related codes?
P0442 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 P0442 is set without the companions, the issue is more isolated.
How do I prevent P0442 from coming back after a repair?
For P0442 (Evaporative Emission System Leak Detected (small leak)), 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.