The 2 NOx readings are reading strange values.

NOx Post SCR(ppm) Reads 2000.004 at startup then after a min or so drops to -40 to -100 (yes negative) same for NOx Pre SCR(ppm)

Is it possible to see the formula that these use? I’m thinking it’s subtracting 100 but since it’s a standard PID I don’t know how to fix it or create a new one that has it adjusted. The 2000.004 is also what it shows when the ignition is off so I’m assuming that’s a infinite or not ready to read.

PS. This is on a 2012 MB GL350 Blutec but I thought the Standard PID’s were supposed to be standard.

I’ve looked at the raw data but it’s not just one byte of the PID and that PID (83) returns 5 bytes of data.

I’ll try logging all five bytes and see if I can reverse the formula.

I logged the raw values of the 5 bytes as well as the results of the PreLoaded NOx readings.

It looks like the first byte is a binary indicator of which of the other bytes contained data. I was getting a constant 3 so with the data I deduced that it was returning 2 values (bit one and two both on) Each of them is a word (2 bytes).
Then reversing the math (with the initial assumption that they were off by 100 got me to the formula of (((B*256)+C))*0.0320440019)-100 with that all of the values match up. I have no idea why it’s scaled by the 0.032.. and then shifted by 100.

5 Bytes are returned A-E

Byte A = Bit indicator
bit 1 = Word one valid (bytes B and C)
Bit 2 = Word two valid (Bytes D and E)

Bytes (B-C) I’m calling it Word 1
(B*256)+C)
This is the Pre SCR NOx

Bytes (D-E) Word 2
(D*256)+E)
This is the Post SCR NOx

I’m just using the raw numbers for now until I can get another way to scale them.

The Built in ones also use ff128a for the Post SCR value but I found that it read exactly the same as using Bytes D-E from 0183.