When coolant gets old it becomes corrosive — either acidic or basic. Depending on water and engine composition, the engine can become a battery with the same or opposite polarity of the car’s battery. Usually, the engine produce negative voltages as high as 1/2 volt negative. This wreaks havoc with all systems, causing irrational computer responses. If the coolant looks bad, replace it! Don’t waste valuable diagnostic time on basic maintenance problems that should already have been addressed! I strongly recommend using distilled or deionized water in the cooling system. And I'll bet most of you never gave the coolant a second thougth.
