P0442 on 2020-2023 Cadillac CT5: Causes and Fixes for a Small EVAP Leak

What's Unique About the 2020-2023 Cadillac CT5

For modern GM vehicles like the Cadillac CT5, the manufacturer has issued technical service bulletins (TSBs) that provide specific diagnostic advice for EVAP leaks. TSB #21-NA-124, for instance, gives technicians a clear diagnostic order for both small (P0442) and large (P0455) leaks: first, inspect all EVAP lines and connections, then investigate the vent valve for malfunction, and finally, check the purge valve. Additionally, a special coverage letter was sent to some 2020 CT5 owners regarding potential failure of the evaporative emissions purge pump, extending the warranty for this specific part to 15 years or 150,000 miles.

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
• Remote start feature may be disabled.
• A faint smell of fuel, though this is less common.
• Difficulty starting immediately after refueling (if the purge valve is stuck open).

Most Likely Causes

1. Loose or Faulty Gas Cap 🔴 High Probability The gas cap is a frequently used component and its seal can wear out, crack, or be improperly tightened, making it the most common cause for EVAP leak codes across all vehicles. Using a non-OEM cap can also lead to sealing problems.
   How to confirm: Inspect the gas cap for any cracks or hardness in the rubber O-ring seal. Ensure it is tightened until it clicks at least once. If the seal is worn, or if tightening it doesn't resolve the code after a few drive cycles, replacement is the next step. An OEM cap is recommended for a proper seal.
   Typical fix: Replace the gas cap with a new, high-quality OEM or equivalent part.
   Est. part cost: $15-$40
2. Failing EVAP Purge Valve/Solenoid 🟡 Medium Probability The purge valve is an electromechanical part that cycles frequently, leading to eventual wear. It can get stuck open or closed, causing a leak in the system. GM TSB #21-NA-124 points to valve malfunction as a key area to investigate. For some 2020 models, GM issued a special coverage letter extending the warranty on the purge pump due to known issues.
   How to confirm: A scan tool can command the valve to cycle. A more direct test is to remove the valve (typically located in the engine bay) and try to blow through it when it's unpowered; it should be normally closed and not allow air to pass. If it's stuck open, it will allow air through and needs replacement. A stuck-open purge valve is a primary suspect if the engine is hard to start after refueling.
   Typical fix: Replace the EVAP purge valve. This is typically located in the engine bay and is accessible for DIY replacement.
   Est. part cost: $20-$50
3. Cracked or Disconnected EVAP Hose 🟡 Medium Probability Hoses in the engine bay are exposed to heat and vibration, causing them to become brittle and crack over time. TSB #21-NA-124 specifically advises inspecting all purge line connections to ensure they are properly seated as the first step in diagnosis.
   How to confirm: Visually inspect all accessible EVAP system hoses and connections between the engine, purge valve, and charcoal canister. Look for small cracks, especially at bends or connection points. The most definitive method is to have a mechanic perform a smoke test, which forces smoke through the system to reveal the exact location of any small leaks.
   Typical fix: Replace the cracked or broken section of hose. Ensure all connections are secure.
   Est. part cost: $10-$60
4. Failing EVAP Vent Valve/Solenoid ⚪ Low Probability This valve is located near the charcoal canister, under the car, making it susceptible to road debris, dirt, and corrosion which can cause it to stick open. While it can cause P0442, TSB #21-NA-124 suggests checking it after inspecting the EVAP lines but before the purge valve.
   How to confirm: A scan tool can command the valve closed during a smoke test. If smoke exits the vent, the valve is not sealing correctly and has failed. This valve is normally open, so a scan tool or direct power source is needed to command it closed for testing.
   Typical fix: Replace the EVAP vent valve. This may involve relocating a filter box per other GM service bulletins to prevent future contamination.
   Est. part cost: $25-$75

Rare But Worth Checking

• Cracked EVAP Charcoal Canister: The canister itself can develop a hairline crack from debris or age, creating a small leak. This is usually found only with a smoke test after all other common components have been checked. Overfilling the fuel tank past the first click can also saturate the canister and contribute to failure.
• Leaking Fuel Tank or Filler Neck: Rust or physical damage can cause a small leak in the fuel tank or the filler neck. This is uncommon but possible, and would be revealed during a professional smoke test.

Diagnosis Steps

1. Read the code with an OBD-II scanner and check for any other related codes.
2. Inspect the gas cap. Remove it, check the seal for cracks or wear, and reinstall it, ensuring it clicks several times.
3. If the gas cap looks old or worn, replace it with an OEM part, clear the code, and drive for a few days to see if the light returns.
4. Per TSB #21-NA-124, perform a thorough visual inspection of all EVAP hoses and lines in the engine bay and underneath the vehicle, looking for obvious cracks, damage, or loose connections.
5. Locate and test the EVAP purge valve in the engine bay. With the engine off, disconnect it and see if you can blow through it. It should be closed. If air passes through, it needs to be replaced.
6. If the previous steps do not reveal the issue, the most effective next step is a professional smoke test. This test fills the EVAP system with a harmless vaporized smoke and makes even the smallest leaks visible.

Parts You'll Likely Need

• Fuel Tank Cap (Gas Cap) (OEM #20915990) — This is the most common failure point. The seal wears out over time, causing a small leak that triggers the P0442 code.
  Trusted brands: ACDelco
  OEM price range: $20-$35
  Aftermarket price range: $15-$25
• Canister Purge Valve / Solenoid (OEM #12666845) — A very common failure item that gets stuck open or closed, causing a leak or flow issue within the EVAP system. It's often the next part to check after the gas cap. GM has special coverage for this part on some 2020 models.
  Trusted brands: ACDelco, Duralast, GM Genuine Parts
  OEM price range: $30-$50
  Aftermarket price range: $20-$40
• Canister Vent Valve / Solenoid (OEM #84524035) — This valve can fail or get stuck, especially due to its location under the vehicle where it is exposed to dirt and debris. It's a known failure point checked after hoses per TSB #21-NA-124.
  Trusted brands: ACDelco, Dorman
  OEM price range: $40-$60
  Aftermarket price range: $25-$45

Related Codes That Often Appear With This One

• P0455 — This code indicates a large EVAP leak. If a component fails significantly, it can trigger both small and large leak codes. The provided TSB #21-NA-124 covers diagnostics for both P0442 and P0455 together.
• P0449 — This code points to a fault in the EVAP vent control circuit. If seen with P0442, it strongly suggests the problem is with the vent valve or its wiring.
• P0496 — This indicates EVAP flow during a non-purge condition, which often points directly to a purge valve that is stuck open.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-124: Diagnostic Tip for Malfunction Indicator Lamp (MIL) Illuminated - DTC P0442 and/or P0455 Set. 🎬 Watch: A breakdown of common causes and fixes for P0442 codes. This bulletin provides a clear, chronological diagnostic procedure for finding EVAP leaks, starting with lines, then the vent valve, then the purge valve.
• GM Special Coverage N212338500: A letter regarding this program extends the warranty on the evaporative emissions purge pump for some 2020 CT5 models with the 2.0L LSY engine to 15 years/150,000 miles due to a known potential for failure.

Platform-Specific Known Issues

• General Motors has issued Technical Service Bulletin #21-NA-124 which provides diagnostic tips for dealers when encountering P0442 and/or P0455. The bulletin advises a systematic inspection in chronological order: 1. Check all EVAP line connections are fully seated. 2. Inspect lines for damage. 3. Investigate the vent valve for malfunction. 4. Investigate the purge valve for malfunction.
• A letter sent to some 2020 Cadillac CT5 owners acknowledges a potential for the 'evaporative emissions purge pump' to fail and extends the warranty coverage for this specific component to 15 years or 150,000 miles. The letter also warns that overfilling the fuel tank 'by more than three clicks' can contribute to this failure.

Mechanic-Grade Diagnostic Values

• Fuel Tank Pressure (FTP) Sensor Voltage — expected: Approximately 1.3V to 1.7V with the gas cap off (at atmospheric pressure). Normal operating range is 0.5V to 4.5V.. Failure: A voltage that is stuck and does not change, or is stuck high (near 4.5V) or low (near 0.5V), indicates a faulty sensor.
• EVAP Vent Valve Solenoid Resistance — expected: Between 20 and 30 ohms across the terminals.. Failure: A reading outside of this range indicates a damaged solenoid winding.

Scan Tool Commands That Help

• GDS2 / Tech2 (GM Dealer Scan Tool): EVAP Service Bay Test — This is a guided test that allows the technician to have the vehicle's computer automatically test the integrity of the entire EVAP system. It commands the purge and vent solenoids to cycle, creating a vacuum to verify the system can seal properly. TSB 21-NA-124 recommends cycling the solenoids with GDS2 during inspection. Note: This test has preconditions, including the fuel level being between 15-85% and a cold engine (coolant temp below ~158°F).

OEM Part Supersession History

• 84524035 (EVAP Vent Valve) → 42933353 — Part has been updated by the manufacturer.
  Heads up: The new part number (42933353) is the correct service replacement for the original. 84524035 also replaced numerous older part numbers like 22865590 and 25932573.

Model Year Variations Within This Range

• 2020: For the 2020 Cadillac CT5 with the 2.0L LSY engine, GM issued a special coverage warranty extension (N212338500) for the 'evaporative emissions purge pump'. The coverage is extended to 15 years or 150,000 miles. This makes the purge valve a key item to check for this specific year and engine, with potential for a no-cost dealer repair.