Reading Time: 6 minutes

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.

Remember, if you ignore the Check Engine light or cover it up with a sticky note, a piece of tape, or a small photograph and wait until there are symptoms before you decide to handle the issue, you could have a long string of issues to deal with. Get that Check Engine light issue handled as soon as possible.

underneath a modern car
If your car’s computer determines a significant leak in the EVAP system, it will trigger code P0440.

What Does the P0440 Code Mean?

Diagnostic trouble code (DTC) P0440 stands for “Evaporative Emission Control System Malfunction – Large Leak.” The caveat is that most vehicles set a different code for a large leak, and don’t even list a P0440 code. Some will set a P0455 or some other code. One of the vehicles that lists the P0440 is a 2007 Jeep Wrangler. Most other vehicles use another trouble code for this condition.

Your car’s computer (the ECM/PCM) will set the code (if it applies to your vehicle and is in the ECM/PCM’s code list) 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, you can read our detailed, technical discussion here about how the ECM/PCM checks for leaks by performing system self tests.

If you want to know the possible causes of the P0440 code, read the next section.

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. If you leave the gas cap off or loose you’ll see this code or an equivalent code, or you may see the fuel cap light mentioned earlier. So check that first.

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.

How the ECM/PCM Checks for Leaks and Triggers OBD2 Codes

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 ECM/PCM checks for leaks by performing system self-tests when certain criteria are met. One of the criteria (there are several) is that the fuel level in the tank must not be too full or too empty, or the test won’t even run. Ford calls for the test to be run if the fuel level is between 15% and 85%, but if it’s more than 85 or less than 15, the test won’t even run. The 2007 Jeep Wrangler uses the following criteria:

gas cap of a vehicle which when broken could trigger code P0440
A damaged or improperly fitted fuel cap can trigger code P0440.

Engine running after a cold start with the difference between ECT and AAT is less than 10° C (19° F). Fuel Level between 12% and 88% full. Manifold vacuum greater than a calculated minimum value. Ambient Temperature between 4° C and 32° C (39° F and 89° F).

Typically, during the “large leak” portion of the test, the PCM closes the normally open vent valve on the open canister port where air enters to purge the vapors. The ECM/PCM then cycles the purge valve to apply a vacuum on the entire fuel tank system. This vacuum should be trapped in the system if there are no leaks and if the vent valve closed the way it should have. The PCM monitors that vacuum via a fuel tank pressure sensor (FTP). Note that despite the fact that this vacuum is supplied from the engine’s intake manifold through the purge valve, this is a very very slight vacuum.

If the system cannot build enough vacuum or the vacuum drops off too quickly after it is trapped and monitored for loss, 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. Some vehicles use a Fuel Tank Pressure (FTP) sensor, which is a three-wire pressure transducer type sensor mounted on the fuel pump plate or somewhere else on the fuel tank. The 2007 Jeep Wrangler uses a two-wire “Evaporative System Monitor” (ESM) switch.

Again, different vehicles check for leaks in different ways. Some vehicles even have an algorithm whereby they check for system leaks before and after fueling and may turn on a light indicating that the fuel filler cap was left loose.

Products Mentioned in this Guide

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

Click a star to rate this article
moog control arm
Notify of
1 Comment
Inline Feedbacks
View all comments

We replaced the gas cap on my wife’s Jetta and that seemed to fix the problem….for a couple of weeks.
The check engine light came back on and I put the coder on it, got a single error code, P0440, and of course this has just happened a week after we got the DMV registration and smog papers in the mail for the Jetta.
It always happens like that, right? smh.
If a new gas cap fixed the problem once but we now have a large gas leak code again, what do you think the problem is? Of all of the possible problems listed above, does anyone have a guess as to which one is most likely to be the problem?

Copyright ©2021, Inc. All Rights Reserved.