P0401 Question

I don't mean this personally at all, but that's an example of the kind of partial-story reasoning you sometimes see when a person has learned one or two things about how something works but hasn't learned the rest.

For example, the EGR flow monitor test (which is how a P0401 code gets set) is made by taking two MAP sensor readings, under different conditions, and subtracting them. So there are lots of ways a MAP sensor could be bad without triggering the code. (Say, if the sensor just always reads so many inHg too high or too low; when one such reading is subtracted from another, the errors cancel and the test result won't be affected.)

Also, the ECM checks for MAP sensor problems (the P0106, P0107, and P0108 codes are for those).

So for a MAP sensor problem to be the cause of a P0401 (but without you seeing also a P0106, P0107, or P0108), it would have to be (a) a MAP sensor problem sneaky enough not to trigger those other codes, and (b) one that is nonlinear enough to not simply be cancelled when the two MAP readings are subtracted.

Which is not to say that a MAP sensor problem can't be the cause, but only to say to approach the diagnosis carefully and get as much data as you can.

 

I'm not sure anybody knows all of them. We know the test runs when the engine is warmed up and you happen to be decelerating from above 25 mph with the engine spinning and the fuel cut off. Then the ECM notes the MAP reading with the EGR valve closed, then opens the valve some, notes the MAP reading then, and subtracts the valve-closed reading. The difference is what's reported as the mode 6 test result.

We know a difference of around 21 kPa (or 6 inHg) is really good, as in a freshly cleaned system, and the trigger for failing the test is much lower (roughly 1 kPa). But we don't know exactly how far the test opens the valve, or whether it's always the same amount or depends on something else, etc. it would probably be pretty hard to effectively duplicate the test by issuing active-test commands in real time during a few seconds of deceleration. You might get a result but who knows how to compare it to a result from the built-in test?

 

The picture of the scanner shows the P0401 as permanent, which means it is not current. The permanent code will eventually clear, so it looks like you may have fixed the problem by installing the new EGR valve.

What are the other two trouble codes that the scanner detected?

 

^^^ Good catch! I looked right at that picture and didn't spot that.

Yes, the permanent code will go away by itself once three consecutive drive cycles have included that self-test and it passed each time, or will go away when you drive the P0401 confirmation pattern once and it passes, if you don't want to wait.

 

Shouldn't be related to the test. The test just gets snuck in now and then during deceleration when the engine isn't doing any work.

 

As covered in a bunch of older threads, it's just a six-terminal unipolar stepper motor.



There's nothing about it that indicates how far open it is. When the car doesn't want EGR at all, it pulses the coils in the right sequence to step the valve toward closed. The valve closes as far as it can and, naturally, doesn't go any further (there's a ski jump that stops it).

Later, when the car wants it to be n steps open, it pulses n steps in the opening direction, having assumed the valve was closed to start with.

 

I'm not sure how often that display polls the CAN bus for the information it displays. I've always regarded it as more toward the "pretty graphics to entertain the passengers" end than the real-time-accurate end of the spectrum.

 

P0456 would pertain to a leak in the evap emissions system.

A more general intake vacuum leak wouldn't give you an evap-specific code like that; you might be seeing P030x misfire codes, P0171 lean mixture, possibly something about MAF sensor range/performance if the leak was big enough.