I was just going to post a brief run through using this MX900 on the Mazda CX-5 that is here with the A/C leak. Bear in mind the system is empty and has a code stored for low refrigerant pressure, P0532.
What I noticed in looking at the internal data for the front body control module is that the ambient (or outside) air temp was -40F which while it is cooler today than yesterday it isn't that cold. One nice thing about this tool is it will use the color red on a data item it thinks you should notice. So even the MX900 didn't think it was really -40 outside today.
The ambient air sensor is in the front grill area and you will see a -40 as the default if the circuit is open so I looked but looks solidly connected.
The PCM also monitors the ambient temp sensor so I wanted to know if it was also seeing -40 because that would affect engine performance, among other things. The only trouble code I had was the refrigerant pressure, the car runs and starts well and I had a feeling the PCM was not seeing -40 when taking all that into account. The PCM was seeing more logical temp from the sensor and no red pid.
Now sometimes you aren't looking at the same data the PCM is seeing because it can make substitutions based on its reaction to an out of range sensor. So to be sure I was looking at the actual data pid and not an enhanced version I went out of the OEM data and entered the global (or generic) OBDII data which by law has to show the actual data, not enhanced. This was also normal temp.
Just what was going on here? Was this a problem internal to the front bcm? Was it a corrupted datastream that only affected one pid? I then noticed the "outside temp" displayed on the cluster was also normal.
Now seemed time to study. On many cars the front body module would be feeding the outside temp display but I wasn't sure on this Mazda. If it worked that way here then the FBCM was very aware of the actual ambient temp but was defaulting to -40 internally for some reason. If the outside temp in the cluster was fed the data from the PCM or directly from the sensor then I was back to a data fault or an internal problem with the FBCM. Thinking about it and reading as much as I could come up with, this is what I decided. Whether the outside temp display relies on data from the sensor, PCM or FBCM I'm not 100% sure but I suspect the FBCM knows the actual sensor data but is altering the data internally to assist it in reacting properly to the low refrigerant. It isn't trusting the data but knows something isn't right so just defaulting to -40 and not dealing with A/C until it is fixed.
It was simply refusing to accept ambient temperature data because the A/C system was empty and the refrigerant pressure sensor was reporting 0 psi.
Thanks for reading.
Kenny@GGAuto.Repair
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