At the end of my wits with P0172 code.

What is your gas mileage right now That's the big question me personally I don't really care about the check engine light or codes because I live in a county and a state that I don't have emissions longhorn turn on lights get sticker and go but I do care if it's affecting my gas mileage I've had an '04 to 0 code now for 2.7 years still get 47 miles to the gallon so I really don't care about that code or the catalytic converter low threshold or any of that because it's not what affecting me or my gas mileage which is why I own the car your results may vary living in high regulated states where everything must be just so. I was just curious whether you're gas mileage was down because of this usually people notice that before they even think about the light being on

 

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Did you purchase OEM or Denso sensors for the dealer, or a reputable parts house?

Three hours is excessive, to say the least. Will they credit the 3 hours towards the repair labor if you buy new sensors from them?

Here is the workup for DTC P0172 ( and DTC P0171) : https://share.qclt.com/%E4%B8%B0%E7%94%B0%E6%99%AE%E7%91%9E%E6%96%AF%E5%8E%9F%E5%8E%82%E8%8B%B1%E6%96%87%E6%89%8B%E5%86%8Cpdf%E6%A0%BC%E5%BC%8F/Repair%20Manual/04pruisr/05/2054m/cip0171.pdf

FYI : you're moderated until you've posted 5 times.

 

That sounds like you've assumed the sensors were wrong and lying about the mixture being too rich.

Which is possible. But it's just as possible the sensors are fine and doing their job and telling you the mixture is too rich.

So that's also worth checking into, especially as you've already come up empty on the sensors-were-wrong path.

 

As part of the 3-hour 'diagnosis' by "the dealership", did they test for unmetered air getting into the system?

 

That would result in the MAF sensor under reporting and a P0171.

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What scantool do you have? My first step is to look at freeze frame data for the code and see when it set- rpm, temperature, fuel trim numbers, etc (record that data).

Next is to monitor ECM scan data when the engine is warmed up (ECT near 180°F). I want to see Short Term and Long Term Fuel Trim, A/F and O2 sensors at: idle, high idle, and steady throttle cruise (when the engine is running).

The upstream A/F sensor is normally around 3.3V, it goes lower (towards 2.0V) when the exhaust gases are "rich", and higher (toward 5.0V) when "lean". Then downstream O2 sensor reads near 1.0V rich, and near 0V lean. Normally is around 0.7-0.8V when engine is hot, running ok and cat is "good". You can use the O2 sensor to "validate" the A/F sensor and FT numbers. (If A/F and FT say "rich", but O2 reads 0V, then something ain't right).

Short Term FT responds to A/F sensor. Goes (+) % when A/F reads lean, and (-) % when A/F reads rich. If STFT goes "too far" away from 0, then Long Term FT makes a correction to bring it back. This correction is stored in ECM memory for that particular combination of RPM & load. (ST + LT = Total fuel trim).

Ideally fuel trim should be near 0 +/- 5%. 10% is "ok". 20% is a problem and likely sets codes.

So get those numbers and report back.

There are only so many possible "causes".
1) the MAF is lying 2)the A/F is lying 3)the injectors flow too much fuel 4)fuel pressure too high 5)purge valve is sucking fuel 6)engine "timing" is off which throws off the expected Volumetric Efficiency.

The data should help guide what path the diagnosis takes to confirm or eliminate some of those possibilities.

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Ignore the P0420 for now- the engine isn't running properly at idle, which is when it performs the cat monitor test.

The system is in control at cruise. Sensors read as expected, the -12.5% LTFT is compensating but it can do it.

Not so at idle. Both sensors report rich but total fuel trim is almost -50%.

Here are known good MAF readings at different conditions- how does your car compare? (these were taken at under 1000ft elevation. If your altitude is much higher, you can adjust your values).



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Have you changed your brake pedal switch at all?

 

Munis, did you get a resolution in the end or are you still stumped...?

 

Pcv valve swapped out even though it still rattled and it's sorted my issues...

 

Code back, so now left my injectors in cleaner for a few days and replaced the injector seals.
Replaced and it's now been a few days and all good so far. I cannot believe that it may have been the injectors all along. Suspect initially the rubbers injector seals at the manifold were dry and cracked. Together with the injectors needing a flush, that's all it may have been.... Engine is on 130k miles only.

 

That shocking on all my 200 plus thousand milers here that I can see out the window none of this has been done I'm pretty much sure of it I have the history of all of these vehicles in their glove box almost every receipt through Toyota warranty and everything is with these vehicles I have not seen anything about injectors o-rings fuel rail or any of that and any of this series of paperworks. 130 k my God you're not even broken good but then again if this is a generation 3 well maybe.