[Centurion]

Hello e90 post,

Recently I had a yellow check engine light. Bmw service told me based on diagnostics, that I had a faulty NOX sensor. My car had done ca. 42 thousand miles (74k km). They also found I had a slightly overheating thermostat (whatever that is) that I should replace as well. I have two questions.

Is this is a sign of potential future trouble with the car (i.e. should I sell the car)?

Can I drive it for a little bit (about 500 miles) with this fault before I have it repaired? (they need to order the parts and then give me a date for the repair, so it might take up to two weeks)


Thank you for your answers!

[Phil325i]

Yes to both your questions. The NOx sensor is an emissions issue only, won't affect how the car drives. If your car overheats, it automatically goes into limp mode so you can't damage it.

[Centurion]

Thank you for the reply. I'm a little worried about my catalytic converter, seeing how they often fail on 320i's and cost a fortune (or around 2000GBP I've heard). Can this issue influence the cat?

[Russbmw]

Yep, and I think it's nearer £300.

Easy to change yourself though if it's any consolation.

You'll need to clear the fault codes and reset adaptations for it afterwards though.

[Phil325i]

NOx sensor is located after the CATs, so shouldn't affect them.

[Centurion]

Thank you.

[4foothill]

Nox sensor affects fuel ratio so it will affect fuel economy, running of the car, and potentially causing more carbon build up over the long run.

Although the Nox sensor doesn't throw a check engine light, so if you're chasing that I'd start with the thermostat.

[Centurion]

@4foothill I thought the check engine light was caused by the NOX sensor - or at least that's what they told me at BMW. Where did you read it couldn't trigger the light?
According to the serviceman the thermostat should be fine for a little bit.

[jamherber]

Originally Posted by Centurion

@4foothill I thought the check engine light was caused by the NOX sensor - or at least that's what they told me at BMW. Where did you read it couldn't trigger the light?
According to the serviceman the thermostat should be fine for a little bit.

On my N53 engined 330i, my NOx sensor had completely failed and the engine light was not illuminated so I would go for the thermostat being the culprit.

[4foothill]

Originally Posted by Centurion

@4foothill I thought the check engine light was caused by the NOX sensor - or at least that's what they told me at BMW. Where did you read it couldn't trigger the light?
According to the serviceman the thermostat should be fine for a little bit.

That blog post mentions it, also a fair few people mention it on here when I was researching it. But mostly because my Nox sensor was screwed but I got no check engine light, just a ton of Nox sensor related codes.

[Centurion]

Well now I'm getting worried - it might be the fabled amber light of doom that no one can explain and people change all sorts of parts (vacuum pump, cam chain etc) to get rid of.

Thank you for your help!

[4foothill]

I really recommend getting the cable yourself and learning how to read and reset codes. Then you can reset your codes, drive around a bit and recheck your codes as soon as you notice issues. It really helps with narrowing down the more important issues and stops the expensive guess work that the dealer's do. PM me if you need a hand with any of it, although there's plenty of guides on the coding forums.

[dave1973]

I had a 320i with a failed NOX sensor & that didn't trigger any warning lights. Plenty of other things did, but not that.

[BimmerBod]

reading these comments, I had a nox sensor issue - with the darn engine light! a real annoying light i wanted to get rid of, anyway changed the nox sensor as I was told thats the issue and guess what?! the car runs so much better...after a week the engine light is back on! Great.

[aaronblack]

Originally Posted by BimmerBod

reading these comments, I had a nox sensor issue - with the darn engine light! a real annoying light i wanted to get rid of, anyway changed the nox sensor as I was told thats the issue and guess what?! the car runs so much better...after a week the engine light is back on! Great.

Nox itself does not cause EML. My NOx sensor was gone so was my cat and neither ever caused EML. The light is definitely something else, need to scan the car and see what it is.

[RBJ2021]

BimmerBod

Did you ever get this fixed? got the same issue on mine. NOX Sensor fault codes causing the Engine light to come on. replaced it and car felt better. Week later light is back on?