355 Slow Down Light fixed, but my numbers are still off

If you have a 5.2 car, yes. There's a temp probe next to the bypass valve, and the ECU for it is in the RR corner of the engine bay next to the R main cat ECU. Make sure the probes aren't swapped on the ECU's.

Incidentally the bypass probe is the one that's giving me fits....totally erratic flashing SDL. I've swapped ECU's, no change. Next I'm going to swap probes, but those rarely go bad.

 

The probes and ECU's are identical for all three. My understanding is that the car's main ECU is looking for different signals from the bypass vs main cats. Bypass is more complicated, because it wants to see temp changes that track with bypass operation, whereas for the main cats, it's just making sure temps don't get too high.

 

I had an issue where I accidentally swapped the rhs cat ecu with the bypass ecu because they are right next to each other and all the cables are identical. Also can get a weird state w test pipes where the temperature is so low it triggers an sdl.

 

Should be the same as when you turned off the car. The 5.2 car doesn't reset the learned parameters with a battery reset and I'm not sure they even start changing until the car warms up.

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The CAT ECUs are for measuring temperature. I'm not sure why you're looking at these. It's your fuel trims you are worried about.

 

Sorry, I missed your mention of the SDL. If it appeared with the car being cold, then you may have an intermittent Cat ECU or thermocouple problem which may return, but it won't be affecting the fuel trims.

Despite your values, it seems that the Engine ECU hasn't decided that these values are something to worry about and set a fault code or illuminated your Check Engine light.

It's up to you whether you want to do some further investigation.

I'm not sure at this point if your scanner is showing an average voltage. I'm hoping the experts will step in here, but normally, the voltages should fluctuate rapidly on the Sensor 1's after the car has warmed up. The voltages should range between 100 and 900mV (the average being around 450mV). Don't ask me why 450 is an average of 100 and 900mV If your scanner is showing average values, then your voltage values seem to be slightly higher than normal (rich condition).

So the LTFT value is showing that the engine is trying to compensate for the rich condition by subtracting fuel. To figure out what is happening to your car, you may have to keep the engine running for a while to see if the STFT value goes back to 0.0%. It should do this if the LTFT is compensating for the excess fuel.

There are probably a number of reasons why the car might be running rich:

Faulty MAF sensor
Faulty Coolant Temperature Sensor
Faulty Air Temperature Sensor
Faulty fuel pressure regulator
Faulty injectors

These items can also make the car run lean, depending on the fault.

I don't know if the split between the LTFT values on the left and right bank is sufficient to help isolate the problem.

If you want to get a good understanding of fuel trims, I recommend these videos:

 

0.19 V on bank 1, sensor 2 could be the result of an exhaust lead up stream of the bank 1 cat.

 

Just to clarify further, the voltage from the pre-cat O2 sensor oscillates (approx. 0.1 to 0.9 volt) and the voltage from the post-cat sensor should be fairly stable and read ~0.1-0.2 volt.

The oscillation of the pre-cat sensor voltage is not really possible to see on a digital voltmeter but an analogue one will show it. Of course, the best it to use an oscilloscope. If the post-cat sensor voltage starts oscillating in a similar manner as the pre-cat one, it means that the CAT is bad. Examples of voltage readings with a good CAT (top graphs) and a bad CAT (bottom graphs):

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Wouldn't an exhaust leak produce more oxygen at the O2 sensor, making the ECU add more fuel, instead of subtracting it?

 

Rear sensor at 0.1 to 0.2 V means too much O2 in the exhaust, assuming the sensor is good.

I was just thinking about this. What I meant was you would have an exhaust leak upstream of the rear O2 sensor but down stream of the front sensor, not up stream of the cat. But with both O2 sensors mounted in the cat housing on a 355 it's I don't see how you could get a leak after the upstream senor and before the down stream unless the cat can is cracked or something. Otherwise, 0.19v at the down steam could indicate a bad cat or bad downstream sensor.

And yes, if the leak was upstream of the front sensor the ECU would see lean and add fuel.