P0401 Question

This is for a gen 3, not the '04? AFAIK it points to insufficient EGR flow. What's the miles, and has EGR been cleaned?

 

How many miles back was the EGR cleaning? Intake manifold done as well?

If all that is good, maybe the EGR valve crapping out, what @ChapmanF calls the "ski jump" getting chewed up, valve not opening properly?

Misfire codes manifest with coolant in combustion chamber?

 

P0401 is an EGR flow code, nothing to do with misfires or DPFE (on a Prius?!) or anything.

The report that it comes back immediately after clearing is also downright weird. The code is set according to the result of an EGR flow self test. The only time the flow self test can run is when the engine is warm and you are decelerating with the engine running but the fuel injection cut off. After a genuine code clear, there's not supposed to be any way for that code to come back until the next time that test can run, which requires some actual driving of the car.

So something in the story isn't adding up.

It would be interesting to use a scan tool to look at the monitor status flags right after doing that code clear. And also to look at the most recent EGR flow test result.

 

This is news.

Maybe should have just continued in this thread:

Head Gasket Failure | PriusChat

 

There is a school of thought here on PriusChat that a reduction in EGR flow (which is what a P0401 code tells you about) may be a factor in head gasket damage. Misfire codes may eventually alert you to a damaged head gasket, but only too late.

I am not one of the strong adherents to that school of thought, but you can definitely read about it here.

 

Intake manifold passages, because they tend to clog sequentially, I'd put lower on the list of suspects. If you're confident recent cleaning was decent, then I'd suspect EGR valve function first.

There might be something useful in attached.

 

Post #6 suggested getting some data, starting with the actual test result of the car's last EGR flow test (it takes the form of a pressure figure, such as in kilopascals, which is really the difference between two MAP readings, one with the EGR valve closed and one with it a certain amount open). That's the easiest thing to start with, and it's also possible to more directly manipulate the valve opening step and observe the raw MAP readings.

I'm not easily thinking of any promising diagnostic approach that wouldn't start with getting some of that info.

 

"Mode 6" monitor results are a standard OBD-II thing that lots of scan tools can do. The thread linked back in post #6 has examples with people not using Techstream.

Active tests are less widely supported.

 

Is that on a different scan tool? The code itself is the only thing the car says. Whatever fortune cookie a scan tool shows you about what the code "means" is just whatever's baked into the tool.

For the record, because P0401 is a code in the P0 range that's standardized by SAE, there is an SAE-standardized fortune cookie for it, which I think is "Exhaust Gas Recirculation 'A' Flow Insufficient Detected".

 

I kinda suspect they all get wet with oil and it doesn't impair their pressure readings.

Those readings don't seem unreasonable to me. Is it close to your nearest weather station's barometer when the engine is off? Is it down somewhere around 4 psi (or 8 inHg) when idling with no EGR?

Are you sure your Innova can't give you Mode 6 results? Innova has a web page about them. Maybe you could check with them about whether your Innova model can show them, and how.

What is Mode 6? – Innova