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According to your profile you have a 91' vette, which means you have a L98 engine if the car is stock.
The #1 & #3 plugs are on the driver's side. Where, in relation to the plugs, is this sensor. On top of the intake manifold, just behind the thermostat housing is the EGR switch, I believe. I can't think of any sensors in the heads or block on my 86' in that location, but I could be wrong, and the 91' could be different.
From: Life is just one big track event. Everything before and after is prep and warm-up and cool-down laps
Cruise-In III Veteran
Cruise-In IV Veteran
St. Jude Donor '12
Could be an extra coolant temp sensor.
The heads are switchable between right and left and someone might have done that and instead of putting in a plug they put in an old temp sensor.