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Error Code P0459 is defined as EVAP Emission System Purge Control Valve Circuit High.
This code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II, especially those made from 1996 up to the present day. Anecdotally, it seems more common among Ford and Subaru vehicle models. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.
For vehicles with EVAP (Evaporative Emission Control Systems), their engine draws in excessive fuel vapors through the gas tank that would otherwise be vented to the atmosphere.
The fuel vapor is routed through a vacuum line to the engine’s intake, while the purge valve or solenoid meters the anticipated level of fuel vapors. All of these are controlled by the PCM (powertrain control module, also known as ECM or engine control module in other makes).
The EVAP canister purge volume control solenoid valve controls the flow rate of fuel vapor from the EVAP canister using an On/Off duty. The On/Off pulses control the EVAP canister purge volume control solenoid valve from the PCM. The longer the On pulse, the more fuel vapor will flow through the valve.
If the PCM detects a high voltage on the canister purge solenoid with the driver commanded off, then the code P0459 is detected.
As with other error codes, this code activates the Check Engine light, which is also its most common and noticeable symptom. This code doesn’t usually cause drivability problems. Other common symptoms include:
- Noticeable fuel odor caused by released fuel vapors
- Increase in fuel consumption
Potential causes for this code include:
- Defective fuse or relay
- Defective purge control valve
- Defective EVAP purge solenoid control
- Open or short circuit engine wiring harness or connector
- Open or short circuit condition purge control solenoid
- Corroded, faulty, loose, open, or shorted electrical components in the EVAP emissions system purge control valve circuit, including wiring or connectors
- PCM/ECM malfunction
How to Check
Diagnosis for this code starts by checking the vehicle using an OBD-II scanner. Technicians will also check for other codes present. In this step, similar codes, or other related codes may be present or show a larger problem that may be addressed first.
After verifying, the next step is to perform a visual check. Mechanics usually proceed on conducting a visual inspection of all electrical wirings and components, to ensure there are no loose or short connections and corroded connectors that are causing the PCM to receive a defective signal.
Next, the mechanic will check the fuel cap. Many times, a poorly installed fuel cap is the cause of this error code.
After performing all the steps above, the code must be cleared, and the system must be retested. If the code comes back to the PCM, then the mechanic will proceed on checking the EVAP emission system purge control valve circuit and should move on to further repairs.
Since the EVAP is only activated during certain driving conditions, it’s important to monitor the control operation of the EVAP control using an advanced scan tool and perform a road test under certain driving conditions required by to activate the EVAP system. Some scan tools require you to activate the EVAP system manually.
Mechanics must check whether the PCM is commanding the EVAP system on. If the system is running properly, it will be important to back probe the PCM wiring harness connector using a graphic multimeter or oscilloscope with a duty cycle feature with the positive lead on the purge control valve pin and the negative lead connected to a known good ground. The duty cycle must be able to match the command from the PCM during EVAP operation. If there is no duty cycle present, then the PCM may be defective.
How to Fix
Common repairs include:
- Repair, cleaning, or replacement of broken, clogged, or defective charcoal canister
- Replacement of defective purge control solenoid
- Replacement of defective vapor hoses
- Replacement of defective vacuum holes
- Replacement of defective pressure/flow sensor
- Replacement or repair of open or shorted EVAP purge control solenoid harness
After checking and conducting the repairs for the electrical, connectors and other components, ensure that the fuel cap is installed properly. It must be in good condition as well. If not, then there’s a good chance it’s what’s causing the problem.
After doing all repairs, the codes must be cleared from the PCM, and the system must be retested. This process lets you eliminate potential causes until you pinpoint exactly what’s causing the problem.