P0496 – Evaporative emission (EVAP) system -high purge flow

By Benjamin Jerew (Contact Me)
Last Updated 2016-09-06
ASE Master Tech

Trouble CodeFault LocationProbable Cause
P0496 Evaporative emission (EVAP) system -high purge flow Wiring, EVAP valve, mechanical fault

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What Does Code P0496 Mean?

The volatility of gasoline has both its pros and cons. Liquid gasoline cannot burn, but the ease at which it evaporates makes it’s vapors useful in gasoline-powered internal combustion engines around the world. On the other hand, since gasoline boils as low as 95 F, and evaporates at even lower temperatures, basically until it freezes, at some -50 F, depending on the exact blend. Because gasoline is composed of a number of compounds harmful to plant and animal life, not to mention the environment, gasoline containers needs to be sealed to prevent vapors from escaping.

The problem is that a perfectly sealed gasoline container, such as the gas tank in your car, wouldn’t work very well. Depending on construction, the fuel pump could eventually deform, collapse, or break a sealed gas tank. Refueling would be difficult as well, because air in the tank would have a difficult time escaping at the same time as new gas is being introduced via the fuel filler tube.

To prevent the aforementioned problems, the evaporative emissions control system, typically referred to as the EVAP system, keeps these things under control. Using a system of valves and an activated-charcoal canister, the EVAP system allows for air to enter, as fuel is being pumped out, as well as for air to exit, as fuel is being pumped in. During refueling, the charcoal canister captures any vapors in the escaping air. Then, while the engine is running, the purge valve allows the engine to burn off any vapors caught in the canister. This way, harmful vaporized hydrocarbons and other chemicals are safely burned, the only result being water and carbon dioxide.

As with all systems, the EVAP system, controlled by the ECM (engine control module), includes a number of self-checks to make sure that it is indeed performing properly, reducing evaporative emissions from making it to the atmosphere. In case of a fault, the ECM will set an evaporative emissions diagnostic trouble code (DTC) and illuminate the MIL (malfunction indicator lamp). One common problem in the EVAP system occurs when the ECM detects a vacuum when there shouldn’t be one, for example, if the purge valve is stuck open. In this case, DTC P0496 – EVAP System High Purge Flow will be set in system memory.

What are the common causes of code P0496 ?

Depending on year, make, and model, DTC P0496 may have number of causes. Here are some of the most common.

  • 1995-2003 Nissan – Certain Nissan vehicles were prone to charcoal canister leaks, which would cause any number of EVAP system DTCs to set. Tiny charcoal particles could get stuck in the Purge Valve, leading the system to erroneously indicate purge problems and leak problems. Solution: Replace the charcoal canister, blow out the purge lines and vacuum lines, clean or replace Purge Valve and other EVAP system valves. (Caution: Do not blow out purge lines or vacuum lines with shop air unless you are sure they are completely disconnected. High pressure can easily ruin other parts of the EVAP system.)
  • Carbon Deposits – Some automakers put the Purge Valve directly on the intake manifold, which can lead to heating and the accumulation of carbon deposits. If enough carbon builds up, it could hold the valve open. Cleaning may or may not be sufficient to restore proper sealing.
  • Poor Installation or Repair – Least common is a short-to-power condition, which may be caused by poor installation of an aftermarket accessory or mis-routing of the harness following a repair.

What are the symptoms of code P0496 ?

Depending on how your vehicle is designed, you may or may not notice any drivability problems concurrent with an EVAP DTC. At the very least, you will experience diminished fuel economy, as the ECM may be running in “limp-home” mode. Concurrent EVAP system DTCs may also be present.

On the other hand, you may notice drivability problems, such as low power, poor acceleration, even misfiring, due to unmetered air entering via the EVAP system. Concurrent DTCs may include System Rich or Lean codes or Cylinder Misfire codes.

EVAP System

How do you troubleshoot code P0496 ?

Determining the cause of excessive purge flow should be relatively straightforward. Since the ECM has determined that there is vacuum in the system when not commanding EVAP Purge, the problem has to do with the purge valve itself or the circuit controlling the purge valve.  A wiring diagram from All Data DIY for your vehicle will be helpful.

  • Purge Valve Check – The easiest way to check for a stuck-open purge valve is to disconnect the EVAP hose while the engine is running. There may be more than one line connected to the purge valve, depending on if the valve is connected directly to the intake manifold or not. One line should always have vacuum when the engine is running. The other side, which connects the purge valve to the charcoal canister, should only have vacuum when the ECM commands it.
    • Start the engine, then disconnect the EVAP hose. If the valve is stuck open, you may be able to hear air sucking into the open port, or you should be able to feel the suction with your fingertip. The engine may start to run erratically, as well, due to the large vacuum leak you just created.
    • If you have a hard time detecting any leak with your ears or fingers, connect a vacuum tester to the open port. Unless the ECM commands it, the Purge Valve should be closed, which means you should measure no vacuum at the open port.
    • If you feel or measure vacuum at the opened port of the Purge Valve (most ECMs block purge flow at idle) disconnect the Purge Valve electrical connector.
  • If the vacuum doesn’t drop, then the problem is in the valve itself, stuck open because of corrosion or contamination. Clean or replace the Purge Valve. Because the Purge Valve is so “inexpensive,” some suggest simply replacing it as a matter of course, but we suggest doing a full diagnosis before condemning it.
    • Pro Tip: Inspect the purge line, as well. In case you find flakes of charcoal, this could mean the charcoal canister has leaked or is leaking, and will need to be replaced. If you do not replace a leaking canister and blow out all the purge and vacuum lines, you may end up with EVAP system DTCs again in a very short time.
  • If the vacuum drops to zero when you disconnect it, then the problem is likely a short to power in the wire harness, which isn’t impossible, but not a common occurrence. Use a DVOM (digital volt-ohm meter) to determine where the short to power is in the harness between the ECM and the Purge Valve.

PURGE_VALVE

Codes Related to P0496

The following are a number of EVAP system codes, which may or may not be found concurrently with DTC P0496:

  • DTC P0440 – Evaporative Emission Control System Fault
  • DTC P0441 – Evaporative Emission Control System Incorrect Purge Flow 
  • DTC P0442 – EVAP Emission Control System Leak Detected (small leak) 
  • DTC P0443 – EVAP Emission Control System Purge Control Valve Circuit 
  • DTC P0444 – EVAP Purge Control Valve Circuit Open 
  • DTC P0445 – EVAP Purge Control Valve Circuit Shorted 
  • DTC P0446 – Evaporative Emission Control System Vent Control Circuit 
  • DTC P0447 – EVAP Emission Control System Vent Control Circuit Open 
  • DTC P0448 – EVAP Emission Control System Vent Control Circuit Shorted 
  • DTC P0449 – EVAP Emission Control System Vent Valve/Solenoid Circuit 
  • DTC P0450 – Evaporative Emission Control System Pressure Sensor 
  • DTC P0451 – EVAP Emission Control System Pressure Sensor 
  • DTC P0452 – EVAP Emission Control System Pressure Sensor Low Input 
  • DTC P0453 – EVAP Emission Control System Pressure Sensor High input 
  • DTC P0454 – EVAP Emission Control System Pressure Sensor Intermittent 
  • DTC P0455 – EVAP Emission Control System Leak Detected (gross leak) 
  • DTC P0456 – EVAP Emission Control System Leak Detected (small leak) 
  • DTC P0457 – EVAP Emission Control System Leak Detected (fuel cap)
  • DTC P0497 – EVAP Emission Low Purge Flow
  • DTC P0498 – EVAP Emission Vent Valve Control Circuit Low
  • DTC P0499 – EVAP Emission Vent Valve Control Circuit High

Other non-EVAP system codes may include:

  • DTC P0170 Fuel Trim (Bank 1)
  • DTC P0171 System too Lean (Bank 1)
  • DTC P0172 System too Rich (Bank 1)
  • DTC P0173 Fuel Trim Malfunction (Bank 2)
  • DTC P0174 System too Lean (Bank 2)
  • DTC P0175 System too Rich (Bank 2)

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