egr problem code p0401

I have a 99 tacoma and i have a engine code p0401 which is insufficient EGR (exhaust gas recirculation) flow. i tested mt egr today at work and it works perfect i checked all the passages and they are all clean. What else can i check to get rid of this engine light because i will clear the code and it comes back 2 days later and i cant stand check engine lights! please help!

 

So many people, including myself, have had this problem. Ive taken my EGR off twice and cleaned it out, I've tested it with a vacuum and it worked perfectly, I've cleaned out all the hoses going into it, the metal hose behind it, and the f***ing light still comes on. I still get about 22 mpg, so I said screw it. The light will randomly turn off for a while, but it ALWAYS comes back on. If you find a permanent solution, let me know!

 

Understanding how the system checks EGR operation is a big part of diagnosing that code. Typical EGR flow codes (Especially the P0401) are not PHYSICALLY measured with a sensor. A electrical problem, whether due to sticking or shorted EGR will usually set a P0406 code (but not always)

To test for a P0401 the computer will monitor your Pre-cat O2 sensor. (Also called your A/F sensor) and when the computer activates the EGR valve it expects to see a specific difference in the signal from the A/F sensor. If it does not see what it expects to see for 2 drive cycles, it will turn on the CEL and you will find the P0401 code. Even if the passages are perfectly clean, you can still get a P0401 code from:

Bad EGR valve (sticking, electronic failure)
Bad A/F sensor (failed sensor, dirty sensor, bad connection)
Internal failure of ECU
Wiring problems (Bad connection, shorted wire, etc)

In the case of a mechanical EGR, vacuum problems can also be a factor (poor quality vacuum, vacuum control, etc)

 

maybe a sensor since its intermittent like that

 

I have the exact same problem with my 2.7, just after I bought it. I removed and cleaned the valve, replaced the modulator (with used) but did not clear the P0401. Finally, I removed the EGR temp sensor (on the intake manifold near the EGR valve), cleaned the tip really good, and re-installed it. This cleared the P0401. Seven months later, the code returned, and I cleaned it again. I can live with this, rather than spend $$$ on a new sensor.