My 2006 X5 M54 with 125K is doing fine, no leak from the heater plastic pipe.

the E39 M54 cars are getting older (compared with E53), and a few people report either the O-rings leak or the plastic pipe cracks a bit, leaking coolant, nothing catastrophic but slow leak.

My question is: those with E53 M54 engine with plastic pipe leak...
1. When did it happen (year/mileage)?
2. What is the mode of failure: leaking O-rings vs cracked plastic?

I am trying to figure out WHEN to do my preventive maintenance.

Thanks!

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT