Excessive EGR Flow
Our emissions expert has put together the following information about the P0402 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.
OBD II Fault Code
- OBD II P0402
Fault Code Definition
- Excessive EGR Flow
- Check Engine Light will illuminate
- In most cases, there are no adverse conditions noticed by the driver
- In some cases, there may be performance problems, such as dying at stop signs or rough idling, hesitation, misfires or lack of power (especially during acceleration), and a decrease in fuel economy
Common Problems That Trigger the P0402 Code
- Excessive Vacuum signal or Electrical signal to the EGR Valve
- The EGR Valve is defective and is opening too far or not properly closing
- Malfunctioning EGR Vacuum supply solenoid
- Lack of proper EGR system feedback to the computer from the:
- Manifold Absolute Pressure Sensor (MAP)
- Differential EGR Pressure Feedback Sensor (DPFE)
- EGR Valve Position Sensor (EVP)
- Ignition System
- Fuel System
- Oxygen Sensor
- EGR Valve
Polluting Gases Expelled
- HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
- CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
NOx gases are formed when the combustion temperature is too high (2500° F). EGR systems are used to reduce the combustion temperature, thus reducing NOx formation.
The exhaust gas recirculation (EGR) system recycles a small amount of exhaust gas from the exhaust system (usually no more than 10 percent) and mixes it with the intake manifold air entering into the combustion chambers. The addition of this inert (or non-combustible) exhaust gas limits the peak combustion temperatures to a range that is below 2500° F, where the formation of nitrogen oxide (NOx) is know to occur. In some cases where the engine is pinging and/or knocking badly from a severe lack of EGR flow, misfires can take place which allow raw hydrocarbons (HC) to be released from the tailpipe.
When the computer sets a code P0402, it means that the EGR flow monitoring criteria has not been met. The EGR monitoring criteria are a set of test values and are usually run during at least two different driving conditions—steady speed freeway driving and steady speed city driving.
The test criteria during EGR operation include:
- Intake Manifold Pressure change
- Amount of change (usually a decrease) in the front Oxygen Sensor(s) signal
- Amount of position change in the EGR valve as measured by an EGR Valve Position Sensor
- Amount of Spark Knock as measured by the Knock Sensor
- Amount of decrease in exhaust back pressure as measured by the Delta or Digital Pressure Feedback EGR Sensor (DPFE)
Code P0402 is often set when the EGR monitoring criteria are excessively triggered—triggers include too much Manifold Pressure change, too much Oxygen sensor change, and too much EGR Temperature change. Code P0402 is often set when the EGR monitoring sensors are still showing EGR flow after the EGR monitoring tests are complete.
P0402 Diagnostic Theory for Shops and Technicians
The code P0402 is often not a problem with the EGR valve itself. Rather, the EGR system is allowing excessive exhaust gases to flow back in to the combustion process or allowing them to flow when they should not, like when the vehicle is idling. Once the code P0402 has been retrieved with a scan tool, the freeze frame data should be documented and analyzed in order to determine what engine conditions were present when the code was triggered. It is recommended that the vehicle be driven in such a way as to duplicate the code setting conditions with a data streaming scan tool connected, so the behavior of the EGR actuating components and feedback sensors can be monitored. Pay close attention to the DPFE and/or EVP signal in the data stream.
Common Tests to Determine Whether the Problem Is an EGR Control Problem, an EGR Feedback Sensor Problem, or a Defective/Sticking EGR Valve
- Raise the engine RPM to about 2000. Raise the EGR valve to its maximum position and then let it suddenly snap back to a closed position. If the idle becomes smoother, the EGR valve may not be closing properly. (Use either a vacuum pump or bi-directional scan tool if it’s a digital EGR valve.)
- Is the EGR valve getting vacuum when it shouldn’t, such as during idle?
- Check the EGR valve for smooth operation throughout its range of motion (either vacuum or digital).
- Test the accuracy of the EGR valve position sensor with a scan tool or DVOM by raising and lowering the EGR valve. Does it show the proper open/closed voltage or percentage?
- Test the Delta or Digital Pressure Feedback EGR Sensor (DPFE) with a data streaming scan tool and verify that the amount of exhaust backpressure voltage or percentage changes according to spec (voltage should rise from about .5 to at least 1 to 3 volts).
- Verify that the front Oxygen Sensor readings drop and that Short Term Fuel Trim increases when the EGR valve opens and then returns to normal when the valve is closed. Short Term Fuel Trim should increase when the valve opens and decrease when the valve closes properly
- Disconnect the EGR valve (either vacuum or electrical type) and test drive the vehicle. Is there any noticeable change or improvement in the vehicle performance?
- Some EGR systems use two vacuum solenoids to supply and bleed vacuum to the valve. If either of these solenoids malfunction, then the valve will be open at times when it should not be, thus causing a P0402 code. Some vehicles use this type of dual vacuum solenoid EGR control.
- Some EGR valves can get a piece of carbon stuck in between the pintle-shaped tip of the valve and its seat, thus causing EGR flow during improper driving conditions. This condition may not set an EGR code, but it may set misfire codes or rich running codes. One way to test for this condition is to test drive the vehicle with a data streaming scanner and study the EGR Position Sensor readings. The readings should go to 0 percent at idle. If not, there may be an obstruction in the valve, but the reading isn't off enough to set a P0402 code. This mostly happens on GM, Honda, and Acura vehicles, but it can happen on any of the Digital EGR-equipped vehicles.