EDIT: Update with Fix -- The problem was the previous owner had screwed up the radiator hose connections downstream of the thermostat. They had connected hose A to hose D, and hose B to hose C at the couplers in the middle. These couplers are located underneath the Coolant Reservoir. Confirmed with scans that normal operating temp is now back to 220-225 regardless of outside temp, and 180-190 with the Sports button on.

Hose A - Lower Radiator Long - long hose with a coupler in the middle
Hose B - Lower Radiator Short - this connects the Hose A to the Thermostat Housing
Hose C - Upper Radiator Short - (Radiator end)
Hose D - Upper Radiator Long - this connects the Hose C to the Thermostat Housing for Manual Transmissions







Anyone ever had a P0128 code where they traced the problem back to something ELSE besides the thermostat, the coolant temp sensor, fan relays, or low coolant levels?

I started getting P0128 code on a new-to me 2012 R60 Countryman S with a manual transmission last fall when the outdoor temps dropped. Scanner confirmed the engine temp was yoyo'ing in the 130 F to 165 F range and the coolant reservoir was full with clean coolant. Seemed like a textbook diagnostic of the original factory thermostat needing replaced at 90K miles. Everyone knows they were junk anyways. Went with a German made MAHLE and the 2-wire harness, new Coolant Pipe to the water pump, and G48 blue coolant. The new redesigned thermostat relocates the Coolant Temp Sensor from an elbow in the radiator hose to the thermostat body. I connected the new wiring harness to the new relocated coolant temp sensor on the thermostat housing, and bled using procedure out of the manual and cleared the code.

Went for a test drive and the scanner showed no improvement in temps. P0128 code came back the next day. Time for more diagnostic work! Re-bled multiple times (heat on/uphill/high rpm/etc) and never saw a single bubble. Unhooked battery to reset computer. Checked fans/fan relays and they are working as expected, coming on high once the P0128 code comes on, stops when the code is cleared. Hard driving keeping the RPM's high with lots of stop/go's, I can get the temp up to around 210 but it immediately drops back down with mellow driving to 130-165. Repeated same results with 2 different scanners. Repeated same results with the wiring harness connected to the old CTS still left in the coolant hose elbow. All testing done with ambient air temps between 30-40 F. New t-stat/CTS must be bad right? Installed a second replacement MAHLE.

Went for a test drive and no improvement in temps. Decoded the MAHLE part number and confirmed it was genuine on MAHLE website. Contacted MAHLE support and after a long talk they said it was an un-diagnosed problem elsewhere in the system and the t-stat/CTS were good. Lower rad hose gets somewhat warm, upper rad hose stays cold until temp hits 210-ish and then both get hot showing the t-stat is opening correctly. Sitting in the garage and idling/revving the engine, temp eventually gets up to 220 and then the thermostat opens and will regulate the coolant temp between 210-220 just fine with the fan cycling on and off at low speed from time to time. Seems to be working just like the MAHLE folks say it should. But the minute I drive away the coolant temps drop back to 130-165 with ambient air temps 30-40 F. Cranking up and turning off the cabin heat in manual mode causes the temp to go up and down, with cabin heat vent temps going up and down with engine CTS readings, so CTS readings seem valid.

In frustration I break down and buy an OEM factory t-stat/CTS. No change. I fabricated a grill bra to cover 90% of the grill like an old diesel truck grill cover. Only a small improvement in temps. Confirmed that the Map Controlled Thermostat connector is hooked up correctly by unhooking it and having a code come up immediately, that self-clears the minute I plug the connector back in.

The thermostat opens two ways. It will open the traditional way when the coolant gets hot, or it can open when the ECU triggers the Map Controlled Thermostat with 12V power to open the coolant flow. Checked that the Map Controlled Thermostat opens up correctly by turning on the Sport button, and the temps correctly drop immediately, but temp isn't controlled at 180-190 like it should. I put my original factory t-stat into a pot of hot cooking oil and confirm the non-Map Controlled part of the thermostat opens and closes at between 210-220.

Everything points to there not ever being any problem with any of the thermostats, coolant temp sensors, or wiring connectors.

Meanwhile, the clutch needed replacing, so I replaced the coolant pump, pulley, and serpentine belt while everything was out and easy to access. Flow tested the radiator and the heater core, and both flow water freely with no obstruction. Used the vacuum method instead of gravity fill this time to fill the coolant. Re-scan at different ambient air temps, and find the engine temps never stay in the correct temp range, even up to 90+ degree outside temps. Any time the engine load drops, the thermostat can't hold engine temps. On a long downhill, engine temps drop to 130's in 90 degree outside temps. But the code itself doesn't trigger in 50+ degree ambient air temps, because the engine hits 170 degrees at some point in every trip. The ECU is happy and won't code as long as the engine hits 170 at any point in a trip, even if the thermostat isn't holding coolant temps correctly.

Don't know where to go next!