Insufficient EGR flow; Pinging/knocking at higher RPMs
When your Toyota gives you a P0401 code, the technical issue is insufficient EGR flow. Like most engine trouble codes, the root cause may or may not be be obvious.
If your engine computer reports a P0401 code, you'll likely notice a pinging or knocking when the vehicle is at higher speeds or the engine is under load.
The exhaust gas recirculation (EGR) system is part of your Toyota's emissions system. It helps to reduce combustion temperatures, as high temperatures cause oxides of nitrogen (NOx) to form, and NOx gases are regulated by vehicle emissions rules (NOx emissions are believed to be the primary cause of smog).
The EGR system has three main components:
- an EGR valve
- an actuator solenoid, and
- a differential pressure sensor (DPFE)
Together, the EGR system is designed to inject just the right amount of exhaust gas (siphoned off the exhaust manifold) into each cylinder prior to combustion to control final combustion temperature and rates. When a P0401 code is tripped, it is because the DPFE sensor found too little recirculation.
There are four likely culprits here:
- First, the DPFE sensor may be faulty or have intermittent circuit problems with its connection to the engine's computer.
- Second, there could be a blockage in the EGR tube leading to the valve - this is usually carbon buildup.
- Third, the EGR valve itself may be faulty or stuck.
- Fourth, there may be a lack of vacuum which is keeping the EGR valve from opening.
The first step in repairing this code is to check the circuit from the DPFE sensor to the ECM ("computer"). Check your Toyota's maintenance manual for the amount of resistance that should be seen and then do a full circuit test.
If that finds nothing wrong, remove the EGR valve and bypass tube and check them for cleanliness and blockages. If they are blocked, follow your maintenance manual's instructions for cleaning them out so you do not damage the valve, then replace them and reset the ECM.
If this does not work, test the EGR valve with a vacuum pump while monitoring engine RPM and DPFE voltage. RPM should change when the valve is open. If not, the valve is either stuck open or not opening because of a loss of vacuum seal. Replace the valve.
Finally, if all these other checks came back OK, it's probably time to replace the DPFE sensor.
Most likely, these steps will result in a repair of the problem.