# Check Engine Light and P21A9



## JPinSTL (Jul 9, 2018)

2018 RS Hatch 6MT Diesel with 15K on it. Just threw this code. Went to Napa for a free scan before heading to the dealer. Their scanner through some generic info with no details, likely an OEM code? One thing I did notice was the DEF level indicator was not reading that day but is showing 35% now. Outside temp was approx 32º so I'm guessing maybe a DEF heater/sensor issue? Anyone seen this? Car seems to run fine, but I'm planning some driving over the coming holiday weekend. I did google and their is a TSB listed with this code but no details. Thanks - JP


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## Blasirl (Mar 31, 2015)

So far all I can find is this:

Reductant Level Sensor B Circuit Range/Performance

and 

A BMW thread suggests that this is related to DEF level sensor

and here is another BMW thread: https://www.e90post.com/forums/showthread.php?t=1245724


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## Ma v e n (Oct 8, 2018)

P21A9 is a rationality check on the reductant level sensor #2, this code means the level sensor was giving unbelievable, but still valid data for more than 1/4 mile. Level sensors, and wiring concerns are the usual culprit here.


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## JPinSTL (Jul 9, 2018)

So after a few starts I'm no longer showing a MIL. It is supposed to be cold again (below freezing) for the next few days so we will see if the code throws again? If it does I'm going to guess its the DEF Heater failed and the sensor is reading incorrectly.


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## JeramiaK (Mar 4, 2019)

Same thing happened to my 2018 Cruze to hatchback with13,568 miles. I took it in and they had no idea what this code was... So next Tues it goes in to be looked at. my DEF fluid level wasn't reading either, but it is on now, so we will see if it stays on till then. Current temps are in 5-20°f area


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