I wanted to measure exhaust back pressure to check for a clogged catalytic converter since my cats are still original. I picked up this gauge from Harbor Freight for around $15: Fuel Pump and Vacuum Tester - https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html. I used an old O2 sensor and drilled/punched out the middle, then JB-welded two of the fittings that came with the gauge to the O2 sensor so I could attach the hose. I bolted the tool into the upstream O2 sensor hole in front of one of the cats and measured basically zero PSI backpressure with the engine running, meaning the cat wasn't clogged. If we put a rag in the exhaust (or I blew into the hose), the PSI went up on the gauge, so I know the gauge was working.