A/F Control test

I've got a DTC P0138, and have already replaced the downstream sensor (aftermarket, I admit)

Using Techstream, when I run the "A/F Control" test, I get the expected values:

-12.5% Upstream = 3.9v, Downstream = .13v
+12.5% Upstream = 2.5v Downstream = .99v

which matches the graph in the manual nicely. However,

0% Upstream = 3.1v Downstream =.87v

At neutral injection, I'd expect both sensors to be "in the middle", although I realize it's a high gain function.
As the data shows, the Upstream sensor is in the middle, but the downstream is still showing "rich".

The freeze data from the DTC also shows a high value (.76) for the downstream sensor.

It appears that either the downstream sensor is crappy, or maybe my catalytic convertor is shot due to my oil burning 215K miles? Opinions?

 

I replaced the PCV valve a few weeks ago, and the old one seemed functional. I didn't check the downstream sensor except under the A/F Control conditions, where it's successfully followed the fuel injection ratio, except to see the "steady" value of .87v at idle. I'll try higher RPM tomorrow morning. My Techstream connection seems to crash a lot (driver problem or OS?) so getting a plot of the voltage is a painful process. Maybe I'll hook up an oscilloscope.

I'll change out the sensor for a genuine Toyota before I replace the CAT.

 

Here's a couple of more data points:
1) The DTC freeze data showed an O2 sensor of .78v
I put the thing into Inspection Mode, warmed up, and found
2) At idle, the sensor is sitting at .83v with a little variation to as far at .78 and .85
3) At 2500rpm, the sensor sits at the same vales as in #2
4) As it returns to idle, the sensor drops to .11 or .13 momentarily, then returns to the .83v

Any clue what this is trying to tell me?

 

It was a plug compatible sensor. Supposedly a Denso, but I think it was marked otherwise. I'll check the connector, but I checked it when I replaced the sensor (and used some contact cleaner). I have another sensor on the way, this time a Toyota part.

Oddly, I got a different, more detailed, message via email that is shown above this post.

 

The original post has the A/F voltage - looks right to me. Haven't gotten any codes besides P0138.

 

Checked the connector and ground connections, and all are per the manual.

OK, here's the latest data. Engine was warmed up and in "inspection mode"

1000rpm a/f=3.22v o2=.76v
1500rpm a/f=3.40v o2=.80v
2500rpm a/f=3.32v o2=.82v
voltages varied within .05 of those voltages over samples

Using the "active test" for the A/F Injection,
0% injection
same values as above

-12.5% injection (manual says to expect a/f greater than 3.35v, o2 less than 0.4v)
1000rpm a/f=3.85v o2=.09v
1500rpm a/f=3.78v o2=.11v
2500rpm a/f=3.75v 02=.09v

+12.5% injection (manual says to expect a/f less than 3.0v, o2 greater than .53v)
1000rpm a/f=2.43v o2=.95v
1500rpm a/f=1.66v o2=.95v
2500rpm a/f=2.27v 02=.95v

It looks to me like both sensors are reacting properly, except that the O2 sensor is "offset" for what it considers rich. I think the o2 sensor switches at about -2% injection, but the software decided to crash right then.

 

Is replacing the A/F sensor as big a pain as the manual indicates? Although I'll do the multimeter test today, and wait for my new O2 sensor before diving into the A/F, I'm inclined to believe it's the forward sensor since my mileage had deteriorated before I quit driving the car as if it was actually running rich.

 

The multimeter test shows the reading is accurate.

Does this theory make sense? :
The A/F sensor would adjust for oil burning, but the burned oil would accumulate in the CAT. The oil sitting in the CAT would "rich-en" the exhaust as it passes through, so the O2 sensor would see "rich", meaning the CAT is the problem.

I think replacing the CAT (49 state style) is cheaper and way easier than replacing the A/F Sensor. I'll do neither until I get the third O2 sensor installed and checked.

 

My third O2 (Toyota branded) sensor works exactly like the original and the non-Toyota replacement. Sits at about 0.8v unless I modify the injection using an Active Test. The new one will go through it's linear point (flip from >.55 to <.04) at about -4% injection.

I'm inclined to think the car is actually running rich, since my MPG had gotten worse over the last year. Replacing the A/F sensor seems like the obvious next step, but I'm missing the manual pages that explain the needed disassembly, so I'm almost ready to throw in the towel.

Open to suggestions!

 

The "parts locator" in the manuals are very frustrating to find actual parts. I think it's on the manifold, since you can replace the whole exhaust system from below, while you need to remove and/or drain a bunch of stuff to get to it. I found the instructions online (along with a few indications that there are any shortcuts).

What I'm really looking for is advice about whether it's the A/F Sensor or the CAT...I guess I could let the dealer give me an estimate, but I think they'd end up shot-gunning parts too.

 

I have to make double sure that the O2 heater is working, then it's either sell the car as-is or dig into it first.

Techstream doesn't offer sensor heater current, although SnapOn's analyzer does (according to Youtube). Guess I have to find a spot where the wire has enough slack to clamp meter it...probably right at the ECU.

 

So oil burning couldn't damage the CAT to create the symptom? I'm skeptical about the A/F Sensor being bad just because it responds as expected and an offset in it's sensor reading seems like an unlikely failure mode.

 

My O2 sensor voltage is not getting lost, at least not between the connector and the ECU. Measuring the A/F is problematic, since I don't think you can get to the connector without removing things.

I don't know here my oil is going - no blue smoke, but oily exhaust pipe. My oil level was suddenly low at about 120K miles, and I think it did some damage. Whether it's leaking through the rings or valve guides, it's getting burned in any case, isn't it?