The NSX Main Relay—commonly referred to as the EFI or PGM-FI relay—is a critical electrical component responsible for powering the ECU and fuel pump during engine startup. Without it, your NSX doesn’t start. Period. And when it’s failing intermittently? That’s when the real headaches begin.
Over time, especially with age and thermal cycling, the internal solder joints of the main relay can develop hairline cracks, leading to intermittent no-starts, engine stalling, or power loss under load. If your NSX cranks but doesn’t fire—or worse, randomly dies while driving—this relay should be one of the first suspects. Owners have reported “phantom” issues where the car appears to be running but loses throttle response, trips a CEL, or temporarily enters fail-safe mode until the ignition is cycled.
That kind of failure mode isn’t just inconvenient—it’s a liability.
Why Replace It?
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Preventative Maintenance: The main relay is a known weak point on 1st-gen NSXs. Waiting until it fails can leave you stranded.
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Heat-Induced Solder Failure: OEM relays are prone to dry solder joints that fail after repeated exposure to under-seat heat soak.
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Intermittent Electrical Gremlins: Power loss to ECU or fuel pump can mimic drive-by-wire issues, CELs, or throttle disconnection. Diagnosing those without addressing the relay first can lead to misdiagnosis and unnecessary spend.
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Cost-Effective Insurance: A new relay is a relatively inexpensive component that can save hours of diagnostics and prevent future breakdowns.
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Plug-and-Play Replacement: No wiring modifications required. OEM spec fitment for all 1991–2005 NSXs.
Key Symptoms of a Failing Main Relay:
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Engine cranks but does not start
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Engine starts intermittently after being parked in hot weather
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Sudden loss of throttle input or limp-mode behavior
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CEL with no stored codes
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Car starts after smacking the panel behind the passenger seat (not kidding—this has actually worked)
Whether you’re running a pristine ’91 or a late-model 2004 NSX, this is a non-negotiable maintenance item once the symptoms appear. Even if your current relay seems fine, consider carrying a spare. Intermittent failures rarely happen in your garage—they happen on the freeway, in the heat, and in the worst possible moments.
Upgrade your NSX’s reliability—replace your main relay before it leaves you stranded.
Fitment: 1991–2005 Acura NSX
Location: Passenger-side interior panel (not behind the seat in later models—common misconception)
Installation Time: ~15 minutes
Tools Required: Screwdriver, trim removal tool, basic socket set
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