Painted steel should work OK. Emissivity is .85 to .90 on the chart I have. If it is flat paint or dirty then it could be even better. I would be curious to see the difference between using the IR gun and what the computer says. It would be helpful to know the difference if any since the IR gun is a good way to go for DIY'rs. In any case probably plenty close enough although I don't really know.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra