manaiauds

Hi to all!

My SES came on for increased emissions. The code that I got was P0128, does anybody know what this code is? Thank you!

BimmerFan52

The code usually stands for "Coolant temperature below thermostat regulating temperature", and a common cause is a failed thermostat heater coil.

The thermostat on the BMW contains a heating coil that can be heated by the computer to accelerate opening of the thermostat (melt the wax) before the water is normally fully heated and hot enough to melt the wax. The computer will run diagnostics upon starting of the engine to check that the coil is in place. If the coil burns out the computer will note that and throw a P0128 code.

Easy to check - remove the harness connection from the thermostat. Place multimeter probes on the two brass conductors that are exposed when the harness is removed and measure the resistance. The resistance of the coil is normally 16ohms. If the resistance reads one then the coil is burned out.

A defective coil will not affect normal cooling of the engine, but just prevent the computer from accelerating the opening of the thermostat. Accelerating the opening of the thermostat is usually called for when a cold engine is pushed hard (not a good idea anyway).

If the coil is defective the thermostat should be replaced. Stay with a Wahler OE type thermostat. I personally have had bad luck with off brands.

If the coil is OK then check back for additional possible causes.

manaiauds

Thanks bimmerfan52. If I will change the thermostat, do I need to change the water pump too?

audiophool

Great answer!

Infinite/open rather. Same as if the DMM probes weren't touching.

BimmerFan52

I neglected to ask you the age of your car. If your car is older than seven or eight years and the coolant system is all original you should strongly consider replacing the key components - water pump, thermostat, coolant reservoir, vent pipe, hoses and belts.

BMW aluminum engines do not like to be overheated, and operating your BMW engine with old cooling system components that grow brittle and crack with age is like playing Russian roulette. If you are caught in a situation where you cannot immediately turn an overheating engine off, gasket integrity can be damaged as the aluminum components over-expand. Replacing a head gasket will be much more expensive than coolant components, and Murphy's Law says disaster will strike on a vacation road trip, leading to expensive towing and a non-negotiable repair bill from an unknown mechanic "if" the parts are even available locally.

In answer to your question - Yes the thermostat alone can be replaced. But make sure you actually measure the thermostat heater coil resistance (or as audiophool correctly points out an open circuit with infinite resistance if the multimeter reads 1). The error code can also be generated if one of the lead wires that runs to the thermostat is broken, as well as other system causes.

Thermostat replacement is about 20 minutes. Key component cooling system replacement is at least a 3-4 hour DIY job given the extensive number of components that need to be removed, but is well worth the peace of mind if your car has reached the bewitching age.

manaiauds

Thank you bimmerfan52, is there a DIY here in the forum for all the components that you just said?

BimmerFan52

There are several good DIYs on the forums - search "water pump" or "coolant system".
I have attached one I used as reference when I did my water pump/thermostat.
Use realoem.com to get PNs.