P0402 is the "Exhaust Gas Recirculation Flow Excessive Flow Detected" code, and it's basically the opposite of a P0401 code. Whereas a P0401 means there is too little exhaust gas recirculation (EGR) happening. A P0402 means there is too much. This results in an overly-rich mixture of oxygen in the fuel/air mix and - as a result - higher engine temperatures.
The exhaust gas recirculation (EGR) system is designed to siphon exhaust gases from the engine exhaust stream and send them back into each cylinder for "re-burning." This re-burning has several effects:
- It reduces the temperature of the combustion process by reducing the amount of oxygen available in the cylinder
- Lower combustion temperatures reduce the formation of oxides of nitrogen (NOx) gasses
- NOx gases are a major contributor to air pollution and smog - reducing them is good for the environment
What The EGR System Consists Of
The EGR system consists of three basic parts: an EGR valve, an actuator solenoid for that valve, and a differential pressure sensor (DPFE) to regulate how much exhaust is being recirculated or "re-burned" in the engine. (More about the exhaust system in this article.) A failure in any one of these components can cause a P0402 code to trip.
Why Does The Code Trip?
When a P0402 trips on a Toyota, it's usually because the EGR valve is stuck open. This may be because something is blocking the EGR valve from closing because the valve itself is damaged, because of excessive vacuum in the valve, or (most likely) because of a faulty DPFE sensor.
Thus, replacing the EGR valve should not be the first thing done to attempt a repair when a P0402 trouble code is given.
Instead, the DPFE sensor should be tested. Check the voltage across the sensor when the car is running against the specification in your Toyota's repair manual. Often, this will confirm the sensor needs replacement.
If not, the next step is to remove the EGR valve and bypass tube and check them for obstructions and/or a vacuum leak. Clean or replace these components as necessary.
For a comprehensive list of trouble codes, check out this article on the Toyota Parts Center Blog.