A P0440 is a generic powertrain code, which means it can appear in any vehicle regardless of make or model. Unlike other OBD-II codes, it will not affect the drivability of your vehicle and has a very low possibility of causing further damage. Nonetheless, you should still address the underlying cause that’s triggering the code.
What Does the P0440 Code Mean?
Diagnostic trouble code (DTC) P0440 stands for “Evaporative Emission Control System Malfunction – Large Leak.” Your car’s computer will set the code when it determines that there’s a significant leak in the EVAP system—however, keep in mind that this doesn’t necessarily mean that there’s a leak.
To help you understand this better, let’s first discuss what an EVAP system is.
An evaporative emission control system (or EVAP for short) typically includes the fuel tank, gas cap, purge valve, vent valve, and EVAP (charcoal) canister. These components work together to prevent fuel vapors from escaping into the atmosphere, and to comply with emissions regulations, the system must be leak-free.
Your car’s primary computer (often referred to as the powertrain control module or PCM) checks for leaks by performing system self-tests when certain criteria are met.
Typically, during the “large leak” portion of the test, the PCM closes the vent valve and cycles the purge valve. Opening the purge valve puts a vacuum on the system and the PCM monitors that vacuum via a fuel tank pressure sensor (FTP).
If the system cannot build enough vacuum or the vacuum drops off too quickly, the PCM determines the system has a leak and sets a large leak code, such as P0440. Usually, the code will be set after the test has failed twice.
It’s worth noting that not all vehicles perform the EVAP system “large leak” test the same way. For example, some vehicles may use a dedicated leak detection pump (LDP) to monitor the pressure drop in the fuel tank.
What are the Possible Causes of the P0440 Code?
- Missing, damaged, or incorrect fuel cap – An improperly fitted or broken fuel cap is the most common cause of an OBD code P0440 being triggered. As soon as you get this code on your OBD scan tool, check your fuel cap to make sure it’s tight and check for visible cracks that may be keeping it from forming a seal.
- Damaged fuel tank filler neck
- Disconnected or punctured EVAP system hoses – Over time, your car’s EVAP hoses may get brittle and become damaged, allowing fuel vapors to leak.
- Faulty gaskets or seals on the fuel tank sending unit – Improperly installed gaskets, especially after completing component repairs, can prevent mating surfaces from forming a tight seal.
- Leaking or damaged carbon canister – Corrosion or even collision damage can cause the carbon canister to crack and fail.
- Faulty EVAP vent valve and/or faulty EVAP purge valve – Stuck valve openings can impede the normal function of the EVAP system, causing a leak to be detected.
- Damaged or cracked fuel tank
- Faulty fuel tank pressure sensor – A defective FTP sensor can send inaccurate data to the engine computer, triggering a P0440 code when it isn’t necessary.
- Faulty LDP pump or switch
What are the Common Symptoms of the P0440 Code?
- Check Engine Light – A wide variety of issues can trigger the Check Engine warning light to illuminate. To accurately diagnose the code, an OBD-II scanning tool must be connected to your vehicle.
- Unusual fuel odors – There are a few instances when drivers may notice the smell of fuel from inside the passenger cabin. This is because a leak in the EVAP system will reduce its ability to contain fuel odors.
Most drivers will only become aware of this issue because of an illuminated check engine light. However, this does not mean that it’s okay to leave this problem unchecked. A vehicle with an EVAP system leak can release harmful pollutants into the air.
How to Diagnose the P0440 Code
To diagnose a P0440 trouble code, the first thing you should do is check the gas cap—if it’s not securely attached, then that could be what’s causing the problem. A loose or faulty gas cap can easily trigger a P0440.
If tightening the gas cap doesn’t work, you may want to try replacing it with a new cap. If this still doesn’t solve the issue, you’ll need to dig further. There are numerous potential causes for OBD-II code P0440, so it can be tricky to diagnose.
For an idea of how to troubleshoot the code, check out the videos below:
How to Fix the P0440 Code
There are multiple reasons why code P0440 might be stored, so unfortunately, you won’t find a specific “magic bullet” fix for the issue—aside from the gas cap tip.
To repair the issue that’s causing the P0440, you’ll need to perform an accurate diagnosis, guided by the videos above, to determine the most appropriate fix. Also, when troubleshooting and repairing diagnostic trouble codes, make sure to consult your vehicle’s factory repair information, as the exact repair measures for a P0440 may vary depending on your car’s make or model.
Repair manuals, such as those from Chilton, are useful, but an ALLDATA subscription is even better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.