P0401 obd2 code and how EGR system works
P0401 obd2 code refers to an egr flow malfunction which needs a complete vacuum inspection. This includes testing vacuum activated solenoids that operate the egr system.
P0401 EGR Flow malfunction
This is what you can do when you see this code: check all the sensors, solenoids related to your egr (exhaust gas regulator valve). Make sure to check your basics like vacuum connections including its presence of vacuum at idle/high idle rpm to the egr valve. Included in the test is the wiring connections to the sensors, solenoids up to the ecm.
You would probably need a good wiring diagram to see this which is available at ATS. Note: if further advance test is required, this would need a scan tool so the vacuum going to the egr valve could be monitored by manually commanding the vsv to open the vacuum flow to the egr valve.
Symptoms of code P0401:
This code can cause your engine to clutter (ping) during acceleration just like the pre-ignition knock. This is noticeable specially when pulling a load or climbing steep grades.
Testing EGR components:
The vacuum supply going to the egr valve can be monitored at idle and at high idle. There should be no change in engine rpm if the egr valve pintle valve is manually lifted up (this is the egr open position or use a hand held vacuum pump connected to egr vacuum port to lift the pintle rod).
Always clean the egr vacuum passage for blockage to make sure the flow is continuous. This can be done by soaking the passage ways with a carb cleaner and blowing them with an air blower. If possible, use a coat hanger to clean hard to access egr passage holes. Finally, it is cheaper to change the DPF egr valve and than changing the egr valve later.
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