If it was coolant the ECU would throw a code. Getting either coolant or any kind of silicate on an O2 sensor will ruin it instantly. If you want to test that, get an old O2 sensor, clamp it in a vice with a voltmeter attatched, then heat it up with a blow torch and test it (normal readings are 0.1V - 0.8V), then spray it with coolant/or coolant smoke, it will either put way WAY lower voltages (0.07 or lower) or will stop completely. This is one reason you never want to use silicate adhesives on your manifold or head gasket, even the slightest ammount will cost you an O2 sensor if it gets in the intake exhaust tract
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