P0401 is a very common OBDII trouble code that’ll pop up in the Toyota Prius. It has to do with the emissions system and stands for Exhaust Gas Recirculation (EGR)→ Insufficient Flow.
The EGR system in your car is responsible for recirculating exhaust gasses in order to lower the vehicle’s emissions output. There are three main parts of the Prius’s EGR system.
- EGR Valve
- Differential Pressure Sensor
- Actuator Solenoid
Toyota Prius P0401 Quick Info
|Definition||Toyota Prius P0234: Exhaust Gas Recirculation (EGR)→ Insufficient Flow|
|Common Causes||Bad EGR Valve, EGR Tube, Faulty EGR Sensor|
|Symptoms||Check Engine Light, Piston Slap|
|Breakdown Risk?||Not Usually|
|Repair Cost (Parts)||Around $100 or less|
Toyota Prius P0401 Meaning
Exhaust Gas Recirculation (EGR)→ Insufficient Flow.
Exhaust Gas Recirculation (EGR)
The EGR system takes exhaust gasses and cycles them through the combustion chamber a second time. This process reduces tailpipe emissions.
The flow of exhaust through the EGR system is too slow. P0401 will not affect the way your Prius runs.
Related: P0420 Toyota Prius
Toyota Prius P0401 Symptoms
Most of the time, there will not be symptoms associated with P0401 in the Toyota Prius. There may be a slight ping or knocking sound on rare occasions.
- Service Engine Soon Light– P0401 will illuminate the service engine soon light.
- Knocking Sound– In very rare cases, a noticeable knock may come from the vehicle.
P0401 Causes: Toyota Prius
Diagnosing what is causing the P0401 code in the Prius is typically not very tricky. Here are the most common causes:
- Blocked EGR Tube– The tube that carries the exhaust gases back to the motor may clog up over time. Alternatively, it can become bent or kinked. This is especially true if you are experiencing P0401 right after doing some engine work. Check the tube and see if it is cracked, clogged, or damaged.
- Faulty EGR Valve– The EGR valve is usually not the place to start.
- Vacuum Issues– The EGR valve relies on the engine vacuum to recirculate exhaust gas. So, if you are encountering P0401 and other codes, the P0401 may be a symptom of larger problems. For instance, if the motor is not running well enough to create a sufficient vacuum to operate the EGR system, P0401 will often be thrown.
- Sensor Issue– The Differential Pressure Feedback- EGR sensor can go faulty and cause the EGR valve to stop operating properly and throw the code.
The good news with P0401 in the Toyota Prius is that it rarely ever costs much money to fix, and it is usually pretty easy to get the problem since it is right on top of the engine.
Most folks see P0401 Insufficient EGR flow, look up the price of an EGR valve, see how cheap it is, and just throw a new one on. That can be a mistake. The EGR valve will not always be the cause of P0401. Here are some of the most common fixes to help clear the P0401:
Check the EGR Tube
It’s a good idea to check the tube going to the EGR. A crimped, broken, or disconnected EGR tube is often the cause of P0401; easy to do, fast, and replacing it is very affordable. If it’s cracked, you’ll need to get a new one. If it’s clogged, you can get away with cleaning it and replacing it.
Check DPFE Sensor Voltage
If you are good with a voltage meter, test the DPFE sensor voltage. Here’s a good article on how to do that from It Still Runs: How to Test DPFE Sensor.
Test the EGR Valve
A vacuum gauge will help determine if the EGR valve has gone bad. Here’s an article from Autozone on how to test it with a vacuum gauge.
Testing your EGR system is relatively easy to do. The most challenging aspect is the DPFE sensor.
One by one, you can eliminate the DPFE sensor and EGR tubing. At that point, it’s likely you’ll have to replace the EGR valve.