NOX Sensor

Plugged my wee OBD reader to my Taxi this morning. Both NOX sensors until engine heats up give maximum readings of 65535ppm which would be normal as car has not got to operating temperature. After a good 45 minutes NOX sensor 2 still reading 65535. Is this normal. The car was driving great too. I have noticed this reading before yet NOX sensor 1 has been totally different. Done a test the other day and NOX sensor 2 showed 0ppm at idle but did increase once i drove off. That would suggest the SCR is doing it’s job however stuck at 0 and at other times 65535 is this a dodgy sensor. Would appreciate if anybody knows more.

65535 is a "magic" number in computer arithmetic, so I'd be suspicious of anything showing it. It's the largest number that 2 bytes of data can store, a bit like 9999 being the largest number that four decimal digits can show. It can also represent -1 - another magic number typically indicating "unknown" if there should actually be a positive number.

I'd expect that anything trying to give a useful figure would be well away from these limits.

Thanks for that. I could try and go down the road of replacing the sensor. But not a cheap way to change it. Ramp job plus £300/400 to replace it. But if the sensor is inaccurate it will need to go anyway.

If the car is running fine then maybe "suck it and see". It's a slippery slope when you go looking for issues!

Obviously, if it develops into something, then get it sorted. If not, then you have saved £300-400!

My P20EE code returned I think on Sunday morning as pending but was gone after 2nd/3rd engine start. I have disconnected the MAF sensor and with no readings from sensors it could well be a sensor causing the issues but what one I don’t know but car drives well with it disconnected. There is no readings from EGR either so is that off when MAF is disconnected. If so then it could be a EGR issue. However all garages I have been to say I should be getting a code regarding the EGR or practically anything.

Checked my scanner on another Passat 2.0tdi on idle at same location as my own car. Practically everything read around the same values which rules out MAF or O2 sensor being the issue. The only thing I did notice differently was the NOX sensor 1 had an average reading of 229ppm compared to my own which is much less which appears to be just over 100 or less than that value. The other Passat is quieter too at idle. Will clean NOX sensor 1 see if there any difference to it.

It's normal for NOx readings to be high until the engine reaches operating temperature, but concerning if sensor 2 stays at 65535ppm even after a 45 min drive. This likely indicates a faulty sensor.

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