Removed battery posts on vehicle to clean and to check water level in battery. Re-connected, started. Message says "Failsafe Mode". Ran vehicle for about 12 miles, failsafe mode gone- but now Service Engine Soon light is on.

Connect it to a professional grade scan tool computer.

Sometimes the engine failsafe mode is triggered due to a sensor failure or an electrical glitch. It's also common for a security system fault to engage this warning light, which means it's not always a mechanical issue that's causing the failsafe.
Oct 31, 2018

What is the Engine Failsafe Mode? ' YourMechanic Advice

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Vehicle Engine Failsafe Mode Explained and How to Reset it

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In most cases, the check engine light will turn ON as the engine switches to failsafe mode. But having a check engine light does not ...
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‎What is the engine failsafe... · ‎Fail-Safe mode isn't always...

P0600 means that the ECU has lost the connection with one of its many controllers more than once. When this code is set the ECU triggers the Check Engine Light to be displayed on the dashboard and a failsafe mode enacted.

The failsafe mode is to prevent further damage and protect the vehicle. Failsafe mode will remain in effect until the normal operation is detected or the vehicle is repaired.

What causes the P0600 code?
1. Failing ECU or the ECU has failed
2. Low battery voltage
3. ECU wiring harness open, short or poor electrical connection

Based on a battery light coming on, I would strongly suspect the battery is failing. Change the battery (or have it tested using a CCA tester, NOT A VOLTAGE TESTER which will (not give you an accurate picture of a battery in a modern car.)

Personally, if this is the original battery, I would change it before considering anything else.

Can be an automatic transmission not working properly check the fluid level disconnect battery for 1 /2 hour.

The thermostat, and radiator are under the hood, not under the dash, you might be sure you have enough anti-freeze, in the radiator and reservoir. if not refill to the proper levels, if it heats up after that, check for leaks, if none exist, you might need water pump, or have a leaky head gasket.

Fail safe is TPS sensor. When you go in fail safe the vehicle give a set amount of RPM and no throttle response the rule of thumb in fail safe is APP1 and APP2 does not agree the goal is to protect the catalytic convertor. So if the transmission was at fault you dont over rev to end up burning the Cat. Or over rev to break the engine It all boils down to a throttle body assembly as the solution

Check engine light on , failsafe mode . The first thing to do would be to hook up a scan tool an check for DTC'S - diagnostic trouble code's . Was the vehicle overheating ? Failsafe mode will shut the vehicle off to save the engine if it is over heating . Do you know if the vehicle has electronic throttle control ? Was their a yellow wrench symbol lit on the instrument cluster as well ?

Trans failsafe is "Limp" mode, it means a sensor has triggered due to a fault in OBC. You need to bring it to BMW or a competent shop to have a fault scan done to identify the actual cause. Limp mode allows you to drive short distance (so you're not stranded roadside)

There is a level sensor inside the coolant recovery tank, it may be defective, a fairly common issue.

I have hearing a lot of this lately with Fords. Most of the time this dash board light comes on due to problems with the Throttle Body (TB)not just your coolant,etc.. If it is sticking,dirty,causing misfires,etc. then this can cause an engine fail safe mode.You might first try putting in some injector cleaner and a higher grade of gas.Many people I know who have had this problem have had to have their TB cleaned and/or replaced. This is something Ford dealerships now about,but hasn't been made a recall or anything.There are service bulletins though.Mainly keeping cleaner in your gas and performing ritual tune-ups/oil changes may be the only thing you can do.Hope this helps

try disconecting the battery for 5 mins see if it reeboots computer.