Definition of the P0440 code
Fuel vapour emission control system malfunction
What P0440 means
P0440 is a generic OBD-II code that indicates that the engine control module (ECM) has detected a leak in the fuel tank vapor system or that the vapor pressure sensor has failed.
What causes the P0440 code?
- The ECM has closed all vapor lines in the fuel tank and is monitoring the vapor pressure and sees a higher pressure drop than allowed within the ECM test parameters.
- The cause of code P0440 may be a loose gas cap, which is the most common cause of the code.
- The P0440 code is also set if any component of the fuel vapor system is leaking, such as the gas cap opening, vent valve, carbon canister, fuel filler neck, or any of the many fuel vapor recovery components.
- The fuel vapor pressure sensor is defective, causing an incorrect reading input to the ECM.
What are the symptoms of the P0440 code?
- The Check Engine light will come on and the code will be stored in the ECM memory.
- You may notice the smell of fuel vapor in or around the vehicle if the vapor leak is large enough,
How does a mechanic diagnose a P0440 code?
- He scans the codes and documents the code in the ECM. He reviews the freeze frame data to see when the fault occurred.
- It checks the entire fuel system, including fuel lines and vapor lines from the front to the rear of the vehicle for any signs of fuel or vapor leaks.
- Checks that the fuel cap is not loose, as this is the most common problem.
- Performs a leak test of the fuel vapor system to try to locate the cause of the vapor leak using the test vapor port.
- If there is no leak, then follow the manufacturer’s test to check the vapor sensor and system in detail for this code, as these systems can be very different between manufacturers.
Most common errors when diagnosing P0440 code
- Assuming the gas cap is leaking without performing a complete diagnosis of the entire system.
- Failing to check with the customer to see if the gas cap has been recently loosened.
How serious is code P0440?
- The P0440 code causes the Check Engine light to come on, and this alone will cause the exhaust emissions test to fail.
- The code means that the fuel vapor system has a vapor leak somewhere in the system and the vapor can be very flammable and dangerous to breathe vapor.
- The loss of vapor in the system just means that the engine is not using the vapor, and this can cause a loss of fuel mileage.
What repairs can repair a P0440 code?
- Tighten or readjust the fuel cap.
- Replacing a leaking fuel cap that has a bad seal or vent in the cap.
- Repair or replacement of leaking steam system components, such as the steam control valve or carbon dioxide absorber.
Additional comments to consider regarding code P0440
The P0440 code is one of the most common codes that cause the Check Engine light to come on in today’s vehicles. The most common cause is that the fuel cap is accidentally left on or underfilled after refueling.
