Thanks to our Partner, NAPA Autotech Training and Pico Technology

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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: [email protected]
• Diagnosing the Aftermarket A - Z YouTube Channel
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