FSR Final stage resistor - part of the HVAC systems (temp and fan) High failure part (like 99% at 10+ years)

Roger that!

I keep a working spare on hand so I'm not stranded. I buy aftermarket versions so I don't expect a full 10 year life. :dunno:

Mike

The goofy "idiot gauge" for the coolant temp can be reprogrammed but you need the coding software and a hack with hex editor to modify the code I guess it's not a user-level option but some brainiac figured out which hex bytes set the low and high limits that snap the idiot gauge to noon. It cost me $100s in gas running rich until finally cold weather made cabin cold to indicate thermostat wouldn't close fully

How do you know 99% have failed in 10+ years?

When I am concerned the radiator temp might be too high or low I check it manually when engine is at operating temp or via the OBC. The gauge is relative. If it moves from a cold start position to somewhere near normal that is what I look for it to be each time I read the gauge. Needle location is specific to my X5. If the needle is not in the normal position I know there could be a problem. I check mileage frequently since decreases, variables considered, often indicate something is not working properly. I don't transfer idiot lights to idiot gauges. They are not on or off. The needle moves to some point and stays. If it doesn't get there or keeps going that is as smart as a temp gauge can be.

The problem is the needle will not indicate a low temp problem. My car was running at 160 vs 190 degrees for 1000s of miles causing all sorts of problems (actual damage even). The needle reads "perfectly ok" over about a 40-50° F range and that is way too big of a range hence it really is a "shit is really broken" function; no clue that things may be slightly off. It's not a helpful gauge. It operates exactly like an idiot light. The dial shows some movement when engine is warming up, not a useful function. The fact the heat won't run until coolant is about 100°F is just as useful. The M54 engine at least will run rich until the coolant temp reaches something like 180F. 20+ higher than the gauge indicates good to go. If they were aligned than it would be great. They aren't so it sucks.

Engine should not run rich once it goes into closed loop which happens when cats reach required temp unrelated to the coolant temp. If your gauge is not functioning properly I suggest replacing the sending unit so that it does. A properly working gauge doesn't function as you describe.

No hard stats on vehicles still titled vs parts sold. Would make an interesting grad student marketing project.....
It only seems that way as most everyone has had a failure sometimes in the E39/E53 life. The funny ones are the ones that fail on in garage queens that are just sitting there on battery tenders. I haven't been bit yet but my personal thoughts are I am on borrowed time as three of my cars just sit on tenders.
Indication of super high replace cycles.
The part is now available as a Chinese knockoff, the dealers near me stock 5 at a time due to high demand on 10+ year old cars, and every shop that tinkers on BMWs locally are experts as they are seeing a few each month around here.
All antidotal elements though. No hard stats just frequency of hits on the DIY articles.

The FSU isn't just an E53 failure point, the E38 and E39 have the same exact problem. When I bought my 540i Touring with 176k miles it needed an FSU really badly because the fan would literally run at max speed and it would turn the car into a refrigerator. Funnily enough the old FSU was from 2004, so it had already been replaced once before (the 540i is a 1999 build). According to the Carfax, my X5 4.6 also had the FSU replaced at the dealership at 80k miles in 2009.

You can add E46s to that list :pullhair:

Don't ask me how I know :rant:

Mike