P0442 on 2019 Buick Regal: Causes and Fixes for Small EVAP Leaks

What's Unique About the 2019-2019 Buick REGAL

For the 2019 Buick Regal, like many modern GM vehicles, the EVAP system components are a known point of discussion. The purge and vent valves, in particular, are common failure items that can cause a persistent P0442 code. The purge valve (ACDelco 12690512, superseded by 12737252) is a notoriously common failure part across the GM lineup, including platform mates like the Chevrolet Malibu. GM has issued a broad Technical Service Bulletin (TSB #21-NA-124) that covers diagnostic tips for this exact code on vehicles from 2000-2023, including the Regal, highlighting that it's a recognized issue across the lineup. The bulletin specifically advises checking EVAP lines first, then the vent valve, and finally the purge valve.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light is on
• A faint smell of fuel, particularly around the rear of the car or near the gas cap.
• In rare cases, a rough idle or difficulty starting immediately after refueling if the purge valve is stuck open.
• Failing an emissions inspection.

Most Likely Causes

1. Loose or Faulty Gas Cap 🔴 High Probability The gas cap is the most frequently handled part of the EVAP system. Its rubber O-ring seal can crack, stiffen, and degrade over time, preventing a proper seal. This is the most common cause for a P0442 on any vehicle and the first thing that should be checked.
   How to confirm: First, ensure the cap is tightened until it clicks multiple times. If the code returns, visually inspect the cap's rubber seal for cracks, stiffness, or deformities. An OEM replacement is often recommended as some aftermarket caps may not seal correctly.
   Typical fix: Replace the gas cap.
   Est. part cost: $15-$35
2. Faulty EVAP Purge Solenoid Valve 🟡 Medium Probability The purge valve is a very common failure point on many GM vehicles, including the Regal and its platform mate, the Chevy Malibu. It can get stuck open or leak internally, causing a constant small vacuum leak that the system detects. A stuck-open purge valve can also trigger a P0496 code. 🎬 Watch: How to diagnose a P0496 code on a 2019 Buick.
   How to confirm: With the engine running and the valve's electrical connector disconnected, use a vacuum gauge on the port leading to the charcoal canister. If you read any vacuum (e.g., 20 in-Hg), the valve is mechanically stuck open and has failed. This type of internal leak will not be found with a smoke test.
   Typical fix: Replace the EVAP purge solenoid valve.
   Est. part cost: $25-$50
3. Faulty EVAP Vent Solenoid Valve 🟡 Medium Probability Located near the fuel tank and charcoal canister, this valve is exposed to road debris and moisture, which can cause it to fail or get stuck. This prevents the system from sealing properly during its self-test. TSB #21-NA-124 suggests investigating this part after checking all lines. Corrosion at the electrical connector is also a common failure point on GM vehicles.
   How to confirm: Diagnosis often requires a scan tool to command the valve closed and then a smoke machine to see if the system holds pressure. If smoke exits the vent valve when it's commanded shut, the valve is faulty. You can also sometimes hear it click when commanded by a scan tool.
   Typical fix: Replace the EVAP vent solenoid valve.
   Est. part cost: $25-$60
4. Cracked or Disconnected EVAP Hose ⚪ Low Probability Over time, rubber and plastic hoses can become brittle and crack. A small, hard-to-see crack in one of the many EVAP lines can cause this code. GM's TSB #21-NA-124 explicitly states to check for properly seated and undamaged EVAP lines as the very first step.
   How to confirm: A thorough visual inspection of all EVAP lines between the engine, charcoal canister, and fuel tank is the first step. The most definitive way to find a small hose leak is with a professional smoke test, which forces smoke into the EVAP system to reveal the leak's location.
   Typical fix: Replace the cracked hose section.
   Est. part cost: $10-$40

Rare But Worth Checking

• Leaking Charcoal Canister: The plastic housing of the charcoal canister itself can develop small cracks over time, leading to a leak. This is less common than valve or hose failure but should be considered if other parts check out.
• Faulty Fuel Tank Pressure (FTP) Sensor: In some cases, the sensor that measures system pressure can send incorrect data to the computer, making it believe there is a leak when the system is actually sealed.
• Cracked Fuel Filler Neck: Though less common on newer vehicles, the metal or plastic pipe leading to the gas tank can rust or crack, especially the surface where the gas cap seals. This can cause a persistent small leak.

Diagnosis Steps

1. Check the Gas Cap: Remove and inspect the gas cap. Ensure the rubber seal is not cracked or brittle. Reinstall it and make sure it clicks at least once. Clear the code and drive for a few days to see if it returns. This is the most common fix.
2. Visual Inspection: Following the advice of GM TSB #21-NA-124, carefully inspect all visible EVAP hoses and lines in the engine bay and around the fuel tank for any obvious cracks, disconnections, or damage. Ensure all connections are fully seated.
3. Test the Purge Valve: Locate the purge valve in the engine bay, typically on or near the intake manifold. With the engine running, disconnect the electrical connector. Then, disconnect the vapor line from the valve and check for vacuum at the valve's port with a gauge. There should be zero vacuum. If vacuum is present, the valve is stuck open and must be replaced.
4. Scan Tool Check: Use an OBD-II scanner with live data capabilities to monitor the Fuel Tank Pressure (FTP) sensor. With the gas cap off, the voltage should be around 1.3V-1.7V. You can also use a bidirectional scanner to command the vent valve closed and listen for a click, which helps confirm electrical function.
5. Perform a Smoke Test: This is the most effective way to find small physical leaks. A smoke machine is connected to the EVAP system service port. The machine fills the system with a harmless, visible smoke under low pressure. Any leaks will be revealed by smoke escaping from the faulty component, such as a cracked hose, a bad seal, or the canister itself.
🎬 Watch: How to find small leaks using a smoke machine.

Parts You'll Likely Need

• Gas Cap — This is the most common failure point due to its frequent use and seal degradation over time. It's the first and cheapest part to check and replace.
  Trusted brands: ACDelco, GM Genuine
  OEM price range: $20-$35
  Aftermarket price range: $15-$25
• Vapor Canister Purge Valve (OEM #12690512) — A very common failure item on GM vehicles. It gets stuck open, causing a persistent leak that triggers the P0442 and/or P0496 codes. This part is located in the engine bay and is generally easy to replace.
  Trusted brands: ACDelco, Dorman, Standard Motor Products
  OEM price range: $30-$50
  Aftermarket price range: $25-$40
• Vapor Canister Vent Valve / Solenoid (OEM #84524035) — This valve is located near the fuel tank and is prone to failure from dirt and moisture, preventing the EVAP system from sealing for its pressure test. It is the second component to check after the EVAP lines, according to GM's own service bulletin.
  Trusted brands: ACDelco, Dorman, Wells
  OEM price range: $30-$50
  Aftermarket price range: $20-$40

Related Codes That Often Appear With This One

• P0455 — Indicates a large EVAP leak. You might see both if a component fails intermittently or the leak is on the borderline between small and large. TSB #21-NA-124 addresses diagnostics for both codes together.
• P0496 — Indicates 'EVAP Flow During a Non-Purge Condition.' This code almost always points directly to a faulty purge valve that is stuck open. Seeing it with P0442 is a very strong indicator that the purge valve is the problem.
• P0449 — This code specifically points to a fault in the EVAP vent valve solenoid circuit. If you have P0442 and P0449 together, the vent valve solenoid is almost certainly the culprit.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-124: Diagnostic Tip for Malfunction Indicator Lamp (MIL) Illuminated - DTC P0442 and/or P0455 Set. This bulletin confirms the commonality of the issue and provides a clear diagnostic hierarchy for technicians.

Platform-Specific Known Issues

• GM Technical Service Bulletin #21-NA-124 provides diagnostic tips for technicians dealing with P0442 and P0455 codes, indicating this is a recognized issue. The bulletin advises a specific diagnostic order: 1. Inspect all EVAP lines for proper connection and damage. 2. Investigate the vent valve for malfunction. 3. Investigate the purge valve for malfunction.

Mechanic-Grade Diagnostic Values

• Fuel Tank Pressure (FTP) Sensor Voltage — expected: 1.3V - 1.7V DC with the gas cap removed (engine on, at atmospheric pressure).. Failure: A reading stuck low (e.g., ~0.3V) can indicate a short in the signal wire or an open in the 5V reference wire. A reading stuck high (e.g., ~5.0V) can indicate an open in the ground wire.
• EVAP Purge Solenoid Coil Resistance — expected: Approximately 15 - 30 Ohms (this is a general value, not manufacturer-specific).. Failure: A reading of infinity (OL) indicates an open coil. A reading of zero indicates a shorted coil. Either requires replacement.
• EVAP Vent Solenoid Coil Resistance — expected: Approximately 20 - 50 Ohms (this is a general value, not manufacturer-specific).. Failure: A reading of infinity (OL) indicates an open coil. A reading of zero indicates a shorted coil. Either requires replacement.

Hidden / Shadow Codes Worth Checking

• Monitor ID $3C, Test ID $81: Mode $06 data for the Engine Off Natural Vacuum (EONV) pressure test results. This shows the data from the pressure-based portion of the small leak test. (see via Advanced OBD-II scanner capable of displaying Mode $06 test results.)
• Monitor ID $3C, Test ID $82: Mode $06 data for the Engine Off Natural Vacuum (EONV) vacuum test results. This shows the data from the vacuum-based portion of the small leak test. (see via Advanced OBD-II scanner capable of displaying Mode $06 test results.)

Scan Tool Commands That Help

• GDS2 / Tech2 (or equivalent): EVAP Service Bay Test — This function forces the ECM to run the entire EVAP leak test sequence on demand, which is used to verify a repair without having to perform a lengthy drive cycle. It requires specific conditions, such as a cold start and holding the engine RPM steady (e.g., 1800-2200 RPM) for several minutes.
• GDS2 / Tech2 (or equivalent): EVAP Vent Solenoid Control — This is a critical bidirectional command used during a smoke test. The technician commands the normally-open vent valve to the 'Closed' position to seal the EVAP system, allowing it to be pressurized with smoke to find physical leaks.
• GDS2 / Tech2 (or equivalent): EVAP Purge/Seal — This function commands the purge valve to create a vacuum in the tank and then seals the system to monitor for vacuum decay. It's a method to test system integrity without a smoke machine, though it may not find all intermittent leaks.

Wiring & Ground Locations

• F32 Underhood Fuse — Located in the under-hood fuse box. On the platform-mate Chevrolet Malibu, this is a 5-amp fuse.. This fuse provides power to the EVAP system components, including the purge and vent solenoids. If there is no power at either solenoid's connector, this fuse should be the first electrical component checked before suspecting a wiring or ECM issue.
• Vent Valve Connector — The electrical connector at the EVAP vent valve, located near the charcoal canister by the fuel tank.. On GM vehicles, this connector is in a location exposed to road salt and moisture. The wires can corrode and break, sometimes inside the insulation right at the back of the connector, causing an open circuit and preventing the valve from operating.

Real Owner Repair Stories

• Ron Ananian The Car Doctor™ YouTube Channel (2019 Buick (model unspecified, but with common GM EVAP setup)) — P0496 EVAP Purge Fault Code (often accompanies P0442)
  ✅ What actually fixed it The diagnosis definitively proved the purge valve was faulty. With the engine running, the mechanic disconnected the valve's electrical connector to ensure it was commanded closed. He then connected a vacuum gauge to the valve's tank-side port and measured ~20 inches of manifold vacuum. Since a normally-closed valve should allow zero vacuum to pass when de-energized, this reading confirmed it was stuck open internally and needed replacement.

"I Checked Everything" — The Actual Cause

• If a smoke test passes (shows no leaks), the P0442 code can still be set by a non-physical leak. One primary cause is a faulty Fuel Tank Pressure (FTP) sensor that is not accurately reporting pressure changes to the ECM, making the computer believe a leak exists when one does not.
• Another possibility is a purge or vent valve that seals properly under the low pressure of a smoke test but fails to hold the vacuum applied by the system during its own self-test (the EONV test). Testing the valves with a hand-held vacuum pump can sometimes reveal leaks that a smoke test misses.

OEM Part Supersession History

• 12690512 → 12737252 — Standard part revision by the manufacturer for design or material improvements.
  Heads up: The new part number (12737252) is the correct service replacement for the original.