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Error Code P0404 is described as Exhaust Gas Circuit Range/Performance.
NOx (Nitrogen Oxide) gas is produced during engine’s combustion of fuel, which is produced from the reaction among nitrogen and oxygen and even hydrocarbons (during combustion), particularly at high temperatures. Big cities which normally has high vehicle traffic, emits large amount of NOx to the atmosphere, and this gas is harmful to the environment, as it contributes greatly to air pollution.
NOx can cause acid and respiratory problems when the engine combustion is too high (2500°F). The Exhaust Gas Recirculation (EGR) plays a big role in the engine’s combustion, through its NOx reduction technique in gasoline and diesel engines. The EGR system works by recirculating parts of the exhaust gas from the engine back to the engine cylinders. Since exhaust gas is inert, it displaces fuel and oxygen, which lowers the temperature of the cylinders (through the EGR valve), so as not to affect the performance of the engine. Too much EGR will keep the engine from idle.
In Error Code P0404, the EGR valve is not vacuum-controller, rather electrically controlled. The valve will also have a built-in feedback system that sends message to the PCM (Powertrain Control Module, the brain of the engine’s electrical system) of what position the valve is in, be it open, close, or in between. The PCM needs this information to whether the valve operates as required.
The Error Code P0404 means the position of the EGR valve is above or below range. Meaning, if the PCM determines that the valve should be operating, but feedback circuit system shows that it is closed, it will trigger the error code. The code is also triggered when the PCM determines the valve should be closed, and the feedback shows it’s open.
- Check Engine light is on
- Engine performance problems, such as pinging on acceleration when driving at high speed or when engine is under load
- Problematic EGR
- High combustion temperature resulting to high NOx emissions
It is common for EGR system to be problematic due to carbon buildup in the intake manifold. This carbon buildup can lodge the EGR valve open when it should be closed. This will cause the engine to idle rough, or not at all. If the valve fails to open, there will be higher combustion temperature, which leads to higher NOx emissions. This however, is barely noticeable for the driver.
Some other causes are:
- EGR passage restriction caused by carbon buildup
- Defective EGR valve, lack of electrical signal or proper vacuum to the valve
- Defective EGR vacuum supply solenoid
- Not enough proper EGR system feedback from MAP (Manifold Absolute Pressure sensor), DPFE (Differential EGR Pressure Feedback sensor), EVP (EGR Valve Position sensor), and EGR Temperature Sensor to the computer
- Faulty EGR Valve Position Sensor or its connectors and/or wiring
- Open or short in ground circuit
- Open or short in the 5V reference circuit
- Open or short in PCM controlled voltage circuit
- Defective PCM (less likely)
How To Check
This error code can be set under regular driving conditions, or when the EGR OBD-II monitored test is performed. To set the test criteria, you will need at least two different driving conditions; steady speed freeway and steady speed city driving. Some monitors use long deceleration on the steady speed data to know whether the EGR monitor can pass properly.
The engine control module determines the right EGR flow in different ways:
- EGR passage temperature increases when it is supposed to be flowing
- The amount of Manifold pressure change must be measurable when it is supposed to be flowing
- Measurable change (normally a decrease) in front of O2 sensor signal
- EGR valve position change measured by the EGR Valve Position Sensor
- Knock Sensor’s measurement of the spark knocks
- Amount of decrease in exhaust back pressure as measured by the Digital EGR Pressure Feedback Sensor
The error code is not a problem on the EGR itself, rather, the EGR valve position sensor sends a message to the PCM, telling it that the combustion process is not getting the right amount of EGR to cool the peak firing temperature enough. Once the code is retrieved by the scan tool, it is important to retrieve the freeze frame data by connecting a streaming scan tool to determine what conditions were present in the engine when it was triggered. The test drive must duplicate the conditions that set code, with the scan tool connected, to monitor the behavior of the EGR valve position sensor.
How To Fix
Use the scan tool to set the EGR valve open, observe the actual EGR position (it will be labeled “desired” EGR position or something similar). The actual position of the EGR should be close to the ‘desired’ position. If it is, then the problem is intermittent, this could mean there’s a lodged piece of carbon, or a bad EGR valve winding, which opens or shorts the valve temperature changes.
If it is not close to the desired position, then unplug EGR sensor, and then check for a good 5V reference voltage to the connector. If doesn’t display the reference voltage, repair the open or short in the 5V reference circuit.
If there’s a 5V reference voltage, then use the scanner to activate the EGR, and then monitor the EGR ground circuit using DVOM (Digital Volt Ohm/Meter). This should indicate a good ground. If it does, then change the EGR valve.
If the valve doesn’t increase incrementally, then fix open or short in the EGR control circuit.
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