57k, full bolt on, STM tuned.

Service Engine Soon light has come on for a second time now in two weeks. Will AutoZone scanning bring up any codes?

Any ideas? Plugs? MAF? o2?

 

P0401 EGR, must be a hose.

 

check hose under intake mani, I had this often with my old car

 

When the engine conditions are right (vehicle speed, throttle position, coolant temperature) the ECU does a test by opening and closing the EGR valve. With the EGR valve open the pressure in the manifold should increase about 0.5 psi. This is measured by the manifold pressure sensor. So, the following things could cause your car to flunk the test.....
1. EGR valve stuck in the closed position or plugged up.
2. As you mentioned, loss of vacuum.
3. Malfunction of the EGR solenoid that supplies vacuum to the valve. This gizmo is located on the manifold stay.
4. Something is wrong with the wiring to or electrical plug on the solenoid.
5. The pressure sensor on the manifold has failed.

 

Mine threw the same code and it ended up being a vacuum line disconnected. I think there are three that come off the EGR but not 100 percent. Worth a check anyhow.