If your truck has california emissions check those valves one in a while, as i quickly learned it could save your truck. Also i got the title wrong, the RPO is K18.
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That, is the Passenger side Checkvalve on my 99 Trailblazer. Being part of the California emissions package as far as im aware of (K18 RPO Air injection pump). my truck already had an exhaust leak, and i had a gut feeling to turn off the metal on the radio (wherever i may roam was the song, had it cranked) and i noticed the exhaust note was different, almost like it was running open manifolds even though the leak was at the catalytic converters forward flange...

I knew what it was as soon as i heard it, and i have never parked a vehicle so roughly and quickly as i did then and never will have to again hopefully. Does anyone have a schematic of this harness (main engine harness, passenger side where it goes down to the airbag sensors, K15 pump and the alternator) and what wire gauges are in it? going to get some 1 inch splitloom and the right gauges of wires and repair the harness as replacing the entire main engine harness is a task i lack the money, time, and sheer brass ***** to do currently. (truck was running when i parked it. so it did not get into the harness too badly, no lights on dash save for fuel light)