My 2007 S auto threw out a P173B code, as read by a O'Reilly's Auto Parts code reader. Interestingly, this code does not show up on any lists. It could be the way the reader they use at O'Reilly's, since someone else got a code of P0128 at one place and P173B at O'Reilly's.

In any case, from the experience of others it is clear that P173B is related to the cooling system-- specifically how the temp sensor and thermostat are performing.

The other symptom I was having was that the fan was coming on for a few minutes after shutdown even after driving a short distance on a cold engine.

Since my car is out of warranty, I thought I would try the cheap stuff first, rather than risk a $600 service bill to replace the thermostat.

Before you order any parts, make sure your coolant level is right. I think it is possible that this code could be thrown if the coolant is too low.

I bought a new temp sensor PN 13627535068 and an O-ring PN 11537547824 from MINI of Atlanta for about $30 including shipping.

The parts and their placement can be seen here:

http://www.realoem.com/bmw/showparts...31&hg=11&fg=35

It is easy to replace. The most time consuming thing may be to drain the coolant first. If you are quick enough, you may not even need to do that, though you'll spill some coolant as you hurry to put the sensor back in the hole.

The first picture shows the two hose clamps you'll need to loosen to remove the air inlet tube. You'll also have to remove the crossover pipe. You can then fold the tube to the left and out of the way without removing the hose that goes into the top of the engine.

The second picture shows the temp sensor in place. Just remove the wire harness and the u-shaped clip and the sensor will pull right out. Put the o-ring on the new one and push it down into the hole, then put the clip back on. Take care with the clip, you don't want to drop it and then try to find it in the engine bay!

Then follow the usual instructions to refill or top up the coolant-- turn on the car (do not start it) and put the heater on high with low fan. Fill the coolant to the Max level. Start the car, leaving the cap loose. Put more coolant in if it drops in the first minute or so. Leave the cap off and let the engine warm up to operating temperature. Then let it cool down completely, and add more coolant if necessary. If the cap is on tight for some reason, do not open it until the engine cools down completely. Opening it under pressure and heat will result in coolant blasting out and scalding you!

Once I had done all this my CEL did not light the next time I drove the car. If I had continued to get the code I would have replaced the thermostat, too, which is a much bigger job.

Hope this helps someone...

 

How did you determine it was the sensor and not a faulty/bad thermostat?

 

His point was....It had to be one or the other. With a 50/50 chance, he decided to start with the cheap and easy repair first (the temp sensor). If the check engine light continued to come on, he would have then moved on to the harder repair. All he was risking was $30 and maybe an hour of his time, so it was a very smart move, IMO.

 

This is interesting. I just got the same code reading at Auto Zone. The tech told me it was a dealer code so I looked it up online and found this thread.

The weird thing is that when I cranked my car up to leave, the engine light was not on any more. I wonder whether that scan at Auto Zone turned off the alert.

Earlier this morning, the car's nav's system indicated "Increased emissions, Emissions-related malfunction. Have the system checked by our MINI dealer as soon as possible."

The car has been slow to heat up. I got gas tonight. I checked the oil yesterday and it appeared fine.

My 2007 MINI Cooper S is not under warranty and I'm trying to keep costs down.

 

May work too, but the 521 thermostat is made of thermal plastic. And while good construction is it sandwiched behind a radiator, above hot hose and in close proximity to a very hot Turbo, not to mention boiling hot fluid has been running through it for probably last 60K miles.....that said, if funds permitted I would think about a new thermostat ( which comes with new sensor and gasket ) and flush/fill.

 

I have a 07 cooper that I just replaced the thermostat on. It stopped the coolant leak, but when I was putting everything together again the female end of the wiring harness that goes to the temperature sensor cracked. I was able to get it back on the sensor as the o-ring sort of held it together. A few days later I got the P173B code. Is there a way to replace the cracked end on the wiring harness or does the entire thing need to be replaced?

 

I was having the same exact trouble as dmeadow, but a day later, the running of the radiator fan stopped. I don't know whether that was due to unplugging the battery or a last air burp in the cooling system.

Here are some potential causes:
1. Trapped air bubble. With all the compartments in the thermostat, this is a likely place, and air would cause heat.
2. Bad coolant heat sensor. This is unlikely with a new thermostat, experts at Pelican told me, but it's possible.
3. Fried sensor plug. This can happen very easily, I read online: you plug the oil pressure sensor into the heat sensor receptacle and vice versa, fire up the engine for a few seconds, and "poof." I'm going to see if I can take an electrical reading of that plug tomorrow.
4. ECU is in trouble, which, thankfully, is EXTREMELY unlikely. A company in Canada will reprogram a junkyard ECU for you for only $499; otherwise, it's about a $2k job.