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This is E39 specific, but it MAY work for the E38 and E53 too - not sure, probably best to find out, before you try it. I made this from info collected off the net, and use it quite a bit, so thought I'd tidy it up and sort it out to hopefully make it of use to others. Because of the high number of problems with cooling systems, and the HUGELY buffered gauge on the E39, I thought I'd make a "How to" to read the KTMP (Coolant Temp) your E39. NEVER trust the gauge - there are a lot of E39's out there with faulty thermostats and are consuming copious amounts of fuel because of running too cold. Also, on those with "map-control" electronic controlled thermostats, they can run too hot if the thermostat heater is burnt out (It logs a fault code in the DME fault memory, but does NOT bring up any warning to the driver). The E39 temp gauge seems to read "normal" (12 o'clock) from about 75C up to at least 115C. High OBC is a text display under your instruments Low OBC is a picture of a car under your instruments Test 7 displays the KTMP figure, in degrees Celcius. This is the coolant temperature that the ECU is measuring in the engine. I've not confirmed it, but some people report change in the KTMP display depending on key position - Position 2 (with the engine running or not) appears to be the side of the coolant sensor used for the ECU temperture readings, and position 1 appears to be the temperature the gauge uses - The sensor is called a "Dual Temperature sender" because it has the two readings. If one side is faulty, this should show it up. To access the "High OBC Secret Menu" and KTMP (Coolant temp) add up last 5 numbers of your vin (mine are 59439, so 5+9+4+3+9 =30) turn ignition on 1 or higher (or start engine) Hold right side button until test appears( 5 seconds?), repeatedly press right button until test 19 is shown, then repeatedly press left button until it says Lock: 30 (or whatever your VIN added up to) press right button press right button until test 07 shows press left button and it will show KTMP - coolant temp. press left will cycle through KTMP, RPM and something else. It will stay there until you turn the car right off (key position 0). To access the "Low OBC Secret Menu" and KTMP (Coolant temp) The 'low' OBC, with your one button is a bit harder: There are three actions you can use with the button: short press, long press (1 second+ish) Wait turn ignition on 1 or higher (or start engine) Press and hold the button until "test 1" appears (5 seconds?) and immediately release the button. quickly start "short presses" on the button, with a short pause between keep up the short presses until you get to "test 19" (if you go past, you can keep going up to 21 and back to 0...) While "test 19" is displayed, "wait" - it will enter test 19 after a short wait (If you accidently drop into another test, Easiest way is to start again - key off and back on) Display should show "Lock ON/OFF", where on and off will alternate between one and the other. short press while it says "Lock OFF" and as soon as it display changes to "test 0", start the short presses again, until you get to "test 7" When "test 7" is displayed, "wait" to enter the test, and it will then display your KTMP. It will stay there until you turn the car right off (key position 0). Thermostat ratings: M52 = 92C, Mechanical thermostat M52TU and M54 = Map Control, Mechanical rating 97C (generally run at 92C - 98C at cruise, may drop down to high 80's briefly under load) M62 and M62TU = Map Control, Mechanical rating 108C (generally run at 92C? - 110C at cruise, may drop down to high 80's briefly under load) S62 = 79C, Mechanical thermostat Diesels = I think 88C, Mechanical Thermostat The Mechanical Thermostats should generally stay above the Thermostat rated temperature, unless going down long gradients using no throttle (lot's of airflow, but no fuel burned) - though that will usually only drop slightly and come backup quickly once fuel is burned in the engine. They may go higher, if the cooling capacity is exceeded - the viscous fan, or auxilliary fan should pull the temps down before "the China Syndrome" The Map Control ones will function like the mechanical thermostats above, except the ECU controls a small heater built into the thermostat. It can heat the thermostat up, forcing it to open more and pull engine temps down during heavy loads. The engine temp can vary a lot, but should never cool down much, for very long. Most of the time you should be in the ranges I stated above. Do be aware, with the Map Control ones: The small heater can burn out, and the ECU loses it's temperature control mechanism - it will them run at the High temperature that is the Thermostats mechanical rating, and should be replaced, ASAP. It will NOT bring up a warning of any kind, except when scanning the DME (ECU) for codes. Diesels may have an EGR thermostat too, which can cause warmup issues. If you have any cooling system issues - use the KTMP reading to see what's really happening - because of the bufferd gauge, it doesn't tell you what's going on, and if it does move to the red, it may already be too late! High OBC cars (only) will give a loss of coolant warning. Low do not. If the coolant suddenly escapes, you can cook the engine without the gauge even moving - the sensors only work properly when they're IN coolant.