Thanks, yes, you’re dead right about the effects of corrosion on circuit resistance (and the weird symptoms that can result); having once lived near the sea, I recall only too well how corrosion can affect circuits as every electrical connection soon turned green.  

I stress I know nothing about you lovely cars, but if your MG cars are like my ugly Rover 100 (MEMS 1.6 version), the coolant sensor would have a negative temperature coefficient (NTC), so when cold it has a high resistance (eg 6k), but at 100C the resistance is only around 100 Ohms, and because Pedro’s ECU thinks his engine is hot even when it’s stone cold, his circuit would be indicating a low resistance.   But I stress I know nothing about your cars or whether or not the coolant sensor has an NTC characteristic.

Going back to your observations about high resistance reminded me of how the sea-air corrosion created a high resistance in the high-current starter circuit.    The braided earth bonding cable between the block and the body glowed and fizzled when the key was turned to the start position but nothing else happened.   The corrosion caused a high resistance, so the cable behaved like an electric heater, before crumbling into brown copper oxide powder.   But that was a very small price to pay for the privilege of living by the Moray Firth.

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