Nope. Coolant was gushing out of the hole when I pulled the old sensor out, and I didn't notice the old washer until after I got the new sensor in and saw the shiny washer had been flushed away onto the transaxle or something. I had lost roughly a cup or two of coolant, and didn't want to lose even more putting the washer back. I put plumbers tape on the threads of the new sensor, and it's not leaking, even after running the engine for a while.
I mopped up the spilled coolant with a rag the best I could, and hosed the rest away with plenty of water.
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