Thanks to our Partner, NAPA Autotech Training and Pico Technology
In this episode, Matt Fanslow takes on a common source of confusion in automotive diagnostics: pressure, vacuum, and the difference between gauge and absolute readings.
What starts as a discussion on PSI and inches of mercury quickly expands into how technicians interpret scan tool PIDs, why definitions matter, and where misunderstandings creep in—especially in the U.S., where we often mix measurement scales. Along the way, Matt also detours (in true Matt fashion) into physics, quantum mechanics, and the origins of universes from near-perfect vacuums.
Yes, really. And yes, it still ties back to cars.
Key Topics Covered:
The two main pressure scales used in the U.S.:
- PSI (pounds per square inch) above atmospheric
- Inches of mercury below atmospheric (vacuum)
Where confusion starts:
- Vacuum gauges vs scan tool data
- PSI vs inches of mercury and how both can technically read positive or negative
Gauge vs Absolute pressure:
- Gauge pressure treats atmospheric as zero
- Absolute pressure references a sealed, near-vacuum chamber
- Why MAP (Manifold Absolute Pressure) sensors often confuse techs
Rules of thumb:
- 2 inHg ≈ 1 PSI
- Atmospheric pressure at sea level ≈ 14.7 PSI (≈ 30 inHg, ≈ 100 kPa)
- Engine vacuum at idle ≈ 18–20 inHg (≈ mid-30 kPa absolute)
How to tell what your scan tool is showing you:
- Quick test: Key On, Engine Off → atmospheric pressure value reveals if it’s gauge or absolute
Metric perspective:
- Why kilopascals (kPa) often simplify things
- Thinking of vacuum as “low pressure,” not “negative pressure”
Physics detour (because Matt can’t help himself):
- Schrödinger’s Cat and quantum absurdities
- Heisenberg’s uncertainty principle
- Absolute zero pressure, absolute zero temperature, and why universes may form from vacuum energy
Why It Matters
A clear understanding of how pressure is defined, displayed, and measured allows technicians to:
- Interpret scan tool PIDs correctly
- Avoid misdiagnosis caused by unit confusion
- Communicate more precisely with peers and customers
- Gain confidence when moving between PSI, inHg, and kPa
Next time someone’s arguing inches of mercury vs PSI on a forum, remember: it’s all about knowing whether you’re looking at gauge or absolute. And if the conversation stalls, just casually mention that universes may have formed from absolute zero pressure. It probably won’t help you win the argument—but hey, it’s a good story.
Thanks to our Partner, NAPA Autotech Training
NAPA Autotech’s team of ASE Master Certified Instructors are conducting over 1,200 classes covering 28 automotive topics. To see a selection, go to napaautotech.com for more details.
Thanks to our Partner, Pico Technology
Are you chasing elusive automotive problems? Pico Technology empowers you to see what's really happening. Their PicoScope oscilloscopes transform your diagnostic capabilities. Pinpoint faults in sensors, wiring, and components with unmatched accuracy. Visit PicoAuto.com and revolutionize your diagnostics today!
Contact Information
- Email Matt: mattfanslowpodcast@gmail.com
Popular Podcasts
On Purpose with Jay Shetty
I’m Jay Shetty host of On Purpose the worlds #1 Mental Health podcast and I’m so grateful you found us. I started this podcast 5 years ago to invite you into conversations and workshops that are designed to help make you happier, healthier and more healed. I believe that when you (yes you) feel seen, heard and understood you’re able to deal with relationship struggles, work challenges and life’s ups and downs with more ease and grace. I interview experts, celebrities, thought leaders and athletes so that we can grow our mindset, build better habits and uncover a side of them we’ve never seen before. New episodes every Monday and Friday. Your support means the world to me and I don’t take it for granted — click the follow button and leave a review to help us spread the love with On Purpose. I can’t wait for you to listen to your first or 500th episode!
Crime Junkie
Does hearing about a true crime case always leave you scouring the internet for the truth behind the story? Dive into your next mystery with Crime Junkie. Every Monday, join your host Ashley Flowers as she unravels all the details of infamous and underreported true crime cases with her best friend Brit Prawat. From cold cases to missing persons and heroes in our community who seek justice, Crime Junkie is your destination for theories and stories you won’t hear anywhere else. Whether you're a seasoned true crime enthusiast or new to the genre, you'll find yourself on the edge of your seat awaiting a new episode every Monday. If you can never get enough true crime... Congratulations, you’ve found your people. Follow to join a community of Crime Junkies! Crime Junkie is presented by audiochuck Media Company.
Cardiac Cowboys
The heart was always off-limits to surgeons. Cutting into it spelled instant death for the patient. That is, until a ragtag group of doctors scattered across the Midwest and Texas decided to throw out the rule book. Working in makeshift laboratories and home garages, using medical devices made from scavenged machine parts and beer tubes, these men and women invented the field of open heart surgery. Odds are, someone you know is alive because of them. So why has history left them behind? Presented by Chris Pine, CARDIAC COWBOYS tells the gripping true story behind the birth of heart surgery, and the young, Greatest Generation doctors who made it happen. For years, they competed and feuded, racing to be the first, the best, and the most prolific. Some appeared on the cover of Time Magazine, operated on kings and advised presidents. Others ended up disgraced, penniless, and convicted of felonies. Together, they ignited a revolution in medicine, and changed the world.