Fuel Delivery
P0442
Evaporative Emission Control System Leak Detected (Small Leak)
P0442 indicates that the PCM detected a small EVAP leak. The opening may be too small to see and can involve the cap, hose, purge valve, vent valve, canister, filler neck, or tank connection.
Diagnostic Snapshot
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Before diagnosing this code, it helps to understand the system behind it.
Read Understanding Fuel Delivery SystemsTechnical Summary
P0442 indicates that the PCM detected a small EVAP leak. The opening may be too small to see and can involve the cap, hose, purge valve, vent valve, canister, filler neck, or tank connection.
Final QA emphasis: P0442 requires correlation of small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing. The code should be diagnosed by reproducing the enable condition, proving loaded electrical integrity, and independently confirming the physical system. OEM thresholds and system design take priority over universal values.
What You'll Learn
- Understand what the PCM is monitoring
- Separate leak, purge, vent, and pressure-sensor faults
- Use scan data and low-pressure smoke testing correctly
- Avoid replacing parts without locating the fault
- Verify the repair through monitor completion
Think Like a Technician
Determine whether the system cannot seal, cannot purge, cannot vent, or cannot report pressure correctly. The code identifies the failed test, not automatically the failed part.
For P0442, ask three questions in order: what did the controller command or observe, can the circuit carry and report that condition correctly, and did the physical system actually respond? Use small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing to identify which layer fails first.
What This Code Means
P0442 indicates that the PCM detected a small EVAP leak. The opening may be too small to see and can involve the cap, hose, purge valve, vent valve, canister, filler neck, or tank connection.
System Overview
The EVAP system stores fuel vapors in a charcoal canister, controls fresh-air entry through a vent valve, and meters vapor into the engine through a purge valve. The PCM monitors the system using valve commands and fuel-tank pressure data.
Why This Code Sets
The PCM commanded an EVAP test and the measured pressure, vacuum, sealing, or flow response did not match expectations.
Common Symptoms
- Check Engine Light or MIL illuminated
- Usually little or no noticeable driveability change
- EVAP readiness monitor may remain incomplete
- Emissions inspection failure possible
- Occasional faint fuel-vapor odor if the leak is near the filler neck, canister, or vapor plumbing
- Related EVAP pressure, purge, vent, or leak codes may also be stored
Most Likely Causes
- 1. loose/damaged fuel cap or filler-neck seal where applicable
- 2. cracked/disconnected EVAP hose or vapor line
- 3. purge or vent valve that does not seal
- 4. damaged canister, tank seal, pump-module seal, or leak-detection hardware
- 5. pressure-sensor/vent-control issue that corrupts leak-test results
- 6. PCM monitor fault only after system sealing and controls are proven
Common Vehicles
- Ford
- GM
- Toyota/Lexus
- Honda/Acura
- Nissan
- Hyundai/Kia
- Dodge/Jeep
Freeze Frame Clues
After Refueling
Check the cap, filler neck, purge valve, and canister condition.
Cold Soak
Many EVAP monitors run only after a cold soak and within a specific fuel-level window.
No Pressure Response
Suspect a major opening, valve fault, unavailable purge vacuum, or inaccurate pressure sensor.
Live Data Expectations
Monitor fuel-tank pressure while commanding purge and vent valves. The pressure signal should respond smoothly and predictably. A flat, erratic, or implausible signal requires sensor and circuit testing.
Typical Verification Tests
- Fuel-cap and filler-neck inspection
- Purge-valve sealing and command test
- Vent-valve command and sealing test
- Fuel-tank pressure sensor test
- Low-pressure smoke test
- Vapor-line and canister inspection
Before You Condemn
Do not condemn the fuel cap, purge valve, vent valve, charcoal canister, fuel tank, or pressure sensor until valve operation, sensor response, sealing surfaces, and smoke-test results are verified.
Before Replacing Parts
- Save freeze-frame data
- Verify fuel level and ambient conditions
- Inspect the cap and filler neck
- Test purge-valve sealing
- Test vent-valve operation
- Verify pressure-sensor response
- Smoke-test the system at approved low pressure
Diagnostic Workflow
- Confirm P0442; save current, pending, history, freeze-frame, readiness, misfire/fuel-trim data where applicable, and the complete powertrain scan before clearing codes.
- Verify the exact definition — Evaporative Emission Control System Leak Detected (Small Leak) — and review VIN-specific system design, enabling criteria, fail-safe strategy, wiring, component locations, bulletins, and test procedures.
- Prioritize companion DTCs and basic prerequisites: battery/charging voltage, module powers/grounds, engine mechanical condition, coolant temperature plausibility, and any shared reference or network faults.
- Recreate the freeze-frame condition safely and graph small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with RPM, load, temperature, vehicle speed, and system voltage.
- Inspect the relevant harnesses, connectors, hoses, pipes, grounds, vacuum lines, fuel/exhaust components, and recent repair areas before disconnecting or replacing anything.
- Compare commanded and actual data. Decide whether the failure is electrical, hydraulic/pneumatic, mechanical, combustion-related, or a sensor-plausibility problem before choosing the next test.
- Perform loaded electrical tests on affected powers, grounds, reference, signal, solenoid, injector, relay, or actuator circuits; static continuity alone is not sufficient for current-carrying circuits.
- Use bidirectional controls where supported to command the affected actuator or system and compare command with current/voltage feedback, sensor response, and actual physical operation.
- Independently verify the physical condition with the appropriate test: calibrated fuel-pressure equipment, smoke/EVAP test, compression/leak-down, ignition waveform, injector balance, airflow/pressure response, or another OEM-approved method.
- Separate bank-specific or cylinder-specific faults from shared-system faults by comparing the opposite bank, neighboring cylinders, redundant sensors, or common feeds under the same operating condition.
- Check for heat-, vibration-, and load-dependent failures by repeating the test as the system warms and by carefully manipulating suspect harness sections while monitoring data.
- Do not condemn a sensor because its reading is abnormal until an independent measurement establishes whether the sensor is wrong or accurately reporting a real physical fault.
- Do not condemn an actuator because it fails to respond until power, ground, control authority, mechanical freedom, and the load side of the circuit are proven.
- Before condemning the PCM, prove stable module powers/grounds, terminal tension, external loads, circuit capability, prerequisite inputs, network integrity, and current calibration/software.
- Repair only the fault supported by test evidence, then perform any required relearn, adaptation, fuel-trim reset, or monitor-specific service procedure.
- Repeat the original enable condition and verify P0442 remains absent current and pending, the monitored data is plausible, driveability is normal, and readiness progresses.
Labor & Inspection Checklist
- Save freeze-frame data
- Verify fuel level
- Inspect cap and filler neck
- Inspect visible vapor lines
- Test purge valve
- Test vent valve
- Verify pressure sensor
- Smoke-test system
- Complete monitor verification
Common Repairs
- Replace or correctly seal a damaged/loose fuel cap when testing proves the cap or seal leaks
- Repair cracked, split, disconnected, or porous EVAP vapor hoses and fittings
- Repair filler-neck, tank, pump-module, or canister sealing leaks found by smoke/pressure testing
- Replace a purge valve or vent valve only when it fails a sealing or command test
- Repair damaged pressure-sensor, purge, or vent wiring when electrical testing proves the fault
- Correct canister damage or saturated/contaminated EVAP components when verified
Shop Notes
- A valve may click electrically and still leak mechanically.
- Never apply unregulated shop air to the EVAP system.
- Repeatedly topping off the tank can saturate and damage the charcoal canister.
- Small plastic leaks may open only with temperature change or hose movement.
Final QA diagnostic boundary for P0442: Evaporative Emission Control System Leak Detected (Small Leak). The technician should center the diagnosis on small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing rather than treating the DTC as a replacement instruction.
Evidence hierarchy: preserve freeze-frame first; confirm prerequisites second; compare command/input with loaded circuit behavior third; verify the physical system independently fourth. This order prevents a biased PID or unloaded continuity test from driving the repair.
Failure-pattern note: separate faults that follow engine load, heat, vibration, bank, cylinder, key cycle, purge/EGR command, or pressure demand. A repeatable pattern is often more diagnostic than a single static reading.
Before-parts rule: sensors require plausibility proof, actuators require power/ground/control plus mechanical-response proof, and modules require external circuit/load proof. Do not use a code description as proof of component failure.
Safety/driveability note: Usually driveable, but the MIL and incomplete EVAP readiness can cause emissions-test failure. Fuel odor or a confirmed liquid-fuel/vapor leak should be treated as a safety issue and repaired before continued driving.
Verification standard: reproduce the original enable condition after repair, not merely a warm idle in the bay. Confirm the monitor's related PIDs remain plausible and that no pending code returns.
P0442 QA case-pattern 1: Reproduce the original operating condition and compare small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0442 QA case-pattern 2: Reproduce the original operating condition and compare small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0442 QA case-pattern 3: Reproduce the original operating condition and compare small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0442 QA case-pattern 4: Reproduce the original operating condition and compare small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0442 QA case-pattern 5: Reproduce the original operating condition and compare small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0442 QA case-pattern 6: Reproduce the original operating condition and compare small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0442 QA case-pattern 7: Reproduce the original operating condition and compare small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0442 QA case-pattern 8: Reproduce the original operating condition and compare small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0442 QA case-pattern 9: Reproduce the original operating condition and compare small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
P0442 QA case-pattern 10: Reproduce the original operating condition and compare small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing with an independent physical or electrical measurement. If the scan value changes but the physical system does not, investigate sensor interpretation, control authority, or a mechanical restriction. If the physical system changes but scan feedback does not, investigate the sensing/reference path. If neither changes, prove power, ground, command, and prerequisite conditions before escalating to the controller. Document the before-and-after data so the repair is verified by evidence rather than by the temporary absence of a code.
Technician Notes
Use only regulated low pressure for EVAP smoke testing. Verify that the vent valve is commanded closed and that the purge valve seals before interpreting smoke-test results.\n\n
Mechanic's Tip
Confirm valve operation and tank-pressure response before replacing parts or interpreting a smoke test.
Common Mistakes
- Replacing the fuel cap without inspecting the sealing surface
- Smoke-testing with excessive pressure
- Failing to close the vent valve during testing
- Replacing the canister before locating the fault
- Ignoring pressure-sensor data
Tools Used During Diagnosis
- Professional scan tool
- Low-pressure EVAP smoke machine
- Hand vacuum pump
- Digital multimeter
- Manufacturer service information
Customer Explanation
The vehicle detected a problem in the system that captures and controls fuel vapors. The cause may be a leak, a valve that is not sealing or moving correctly, a blocked vapor path, or a pressure-sensor problem. Testing identifies the actual fault before parts are replaced.
Frequently Asked Questions
What does P0442 mean?
P0442 is Evaporative Emission Control System Leak Detected (Small Leak). The monitor is focused on small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing.
Does the code prove a part has failed?
No. It identifies a failed monitored relationship. Electrical integrity, physical system behavior, and sensor plausibility must be separated before parts are replaced.
Is it safe to drive with P0442?
Usually driveable, but the MIL and incomplete EVAP readiness can cause emissions-test failure. Fuel odor or a confirmed liquid-fuel/vapor leak should be treated as a safety issue and repaired before continued driving.
What should be checked first?
Save freeze-frame and the complete scan, verify the exact system design by VIN, then reproduce the failure while comparing commanded data with actual electrical and physical response.
What freeze-frame clues matter most?
RPM, load, vehicle speed, coolant temperature, system voltage, fuel trims, pressure/airflow data, command state, and companion DTCs show whether the failure occurs at idle, startup, cruise, acceleration, deceleration, or high load.
What live data is most useful?
Graph small EVAP leak integrity using tank-pressure change, purge/vent sealing, cap/filler sealing, hoses, canister, valves, and smoke/pressure testing plus system voltage, RPM/load, temperature, and relevant redundant or opposite-bank/cylinder data. Look for the first relationship that breaks.
Can another system create this code?
Yes. Shared voltage, airflow, fuel delivery, exhaust, vacuum, mechanical, or combustion faults can make the named monitor fail even when the named component is functional.
Should I replace the most common part first?
No. Common-failure statistics are useful for prioritizing inspection, not for skipping tests. Expensive or invasive parts should have at least two consistent forms of evidence.
When is professional equipment justified?
Enhanced graphing data and bidirectional controls are strongly recommended. Depending on the code, a pressure transducer/gauge, scope, current clamp, smoke machine, injector balance equipment, or compression/leak-down tools may be justified.
How do I verify the repair?
Repeat the original operating condition, confirm electrical and physical results agree, check current and pending codes, and complete enough of the drive cycle to show stable data and readiness progress.
Related Atlas Resources
Related Atlas Resources
Understanding Fuel Delivery Systems
Related Reference Chapters
Diagnostic Confidence
High when valve testing, pressure data, and low-pressure smoke testing are used together.
Explore Related Atlas Resources
Related Diagnostic Codes
- P0441 — Evaporative Emission Control System Incorrect Purge Flow
- P0455 — Evaporative Emission Control System Leak Detected (Gross Leak/No Flow)
- P0440 — Evaporative Emission Control System Malfunction
- P0444 — Evaporative Emission System Purge Control Valve Circuit Open
- P0411 — Secondary Air Injection System Incorrect Flow Detected
- P0412 — Secondary Air Injection System Switching Valve A Circuit Malfunction
- P0413 — Secondary Air Injection System Switching Valve A Circuit Open
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