OBD-II Trouble Codes

P0440 Code: Evaporative Emission Control System Malfunction – Large Leak

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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.

underneath of 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.” 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?

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

What are the Common Symptoms of the P0440 Code?

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.

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In the Garage with CarParts.com is an online blog dedicated to bringing DIYers and devoted car enthusiasts up to date with topical automotive news and lifestyle content. Our writers live and breathe automotive, taking the guess work out of car repairs with how-to content that helps owners get back on the road and keep driving.

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