An O2 sensor plug is a threaded cap that seals an unused oxygen-sensor port in an exhaust system so no exhaust leaks out.
Exhaust systems don’t tolerate open holes. When an oxygen sensor comes out of a header, downpipe, or decat pipe, the bare port becomes a leak path for pressurized exhaust gas. An O2 sensor plug is a simple threaded cap that screws into the sensor port. Most come with a copper or aluminum crush washer, and the plug is tightened so the washer compresses and seals the joint. Screw-in installation means no welding, no special tools, and no exhaust downtime beyond the few minutes it takes to swap the part.
What An O2 Sensor Plug Does
An O2 sensor plug blocks an unused oxygen-sensor port, keeping exhaust gas from escaping through an open hole. It is not a sensor, a resistor, or an electronic part — it is purely a mechanical seal. Ports end up unused in two common situations: aftermarket exhausts, decat pipes, and downpipes add sensor bungs for multiple vehicle configurations, leaving at least one empty; and when an oxygen sensor is removed for testing or tuning and never goes back in, that empty hole needs a plug.
Replacing one is straightforward because the plugs are standard catalog parts. Walker’s Oxygen Sensor Bung Plug 35299 is stocked at O’Reilly Auto Parts, Dorman’s 917112 Oxygen (O2) Sensor Plug / Bung is at RockAuto, and JEGS carries its own steel and stainless-steel plugs in 18mm. If you’re choosing one rather than matching an original part, the best O2 sensor plug options we’ve tested sorts the picks by thread size and material.
Will Any O2 Sensor Plug Fit Your Exhaust?
No plug fits every exhaust — the thread must match the bung exactly, and a universal fit listing still assumes the correct thread size and port style. The two standard sizes are M18 × 1.5, the size most listings show, and M14 × 1.5. Outside those, exhaust shops weld in a bung to match either thread — that’s why JEGS’s O2 sensor plug listings pair plugs with weld bungs and fittings for custom exhausts.
The table below is the quick reference for what you’re buying.
| Spec | What To Know |
|---|---|
| Thread sizes | M18 × 1.5 (most common) or M14 × 1.5 — match the bung exactly |
| Materials | Steel, stainless steel, nickel-plated steel, or aluminum |
| Sealing | Copper or aluminum crush washer reduces leak risk |
| Typical uses | Aftermarket exhausts, decat pipes, downpipes, unused sensor bungs |
| Fit | Universal claims are limited; thread size and port design decide fit |
| Main failure mode | Thread mismatch, which strips or cross-threads the bung |
| Typical price | JEGS singles run $4.99–$8.99; two-packs $17.39–$19.09 |
If you aren’t sure which size you need, check the threads on the sensor you removed or measure the bung opening. Guessing is the one move that costs money — a wrong plug won’t start cleanly and can damage the port.
Installing One: The Step Order That Works
Installation is simple: confirm the thread, screw the plug into the sensor port, and tighten securely. The crush washer does the sealing, so the plug itself is the easy part.
- Confirm the thread. Match the plug to the bung before you insert anything. If the plug won’t start cleanly, stop — forcing the wrong thread is how bungs get ruined.
- Clean the port threads so the plug and washer seat against metal, not soot and rust.
- Insert the plug into the oxygen-sensor port and tighten it securely with a wrench.
- Leak-check at idle. Start the engine and listen near the port — no hiss and no exhaust smell means the seal is good.
One common mix-up is worth naming: an O2 sensor plug is not an O2 sensor electrical connector. The plug seals an exhaust port; the electrical connector joins the sensor to the vehicle’s wiring harness. They are unrelated parts that happen to share the O2 name.
The leak you’re preventing is the whole point. Remove an oxygen sensor without installing anything, and the exhaust is left with an open hole that can create leakage — noise, fumes, and a leak that can affect how the engine runs. A plug is the right follow-up, and it costs less than a single exhaust repair. That’s the whole job: threaded cap, crush washer, two common thread sizes, and a quick install. Order the right thread, keep the washer in place, tighten securely — and the port stays sealed.
FAQs
Why does an O2 sensor plug need a crush washer?
Most plugs ship with a copper or aluminum crush washer, and that washer is what actually seals the joint. Without it, the metal threads alone can let exhaust gas seep past. Keep the washer seated when you tighten the plug, and make sure it’s the right washer for the thread size.
Will an M18 plug work in an M14 bung?
No. Thread size must match exactly. An M18 × 1.5 plug fits an M18 × 1.5 bung, and an M14 × 1.5 plug fits an M14 × 1.5 bung. Forcing a mismatched plug strips the bung threads and turns a simple job into a weld-in repair.
What happens if you remove an O2 sensor and leave the port open?
You’ve left an open hole in the exhaust, and that hole can create an exhaust leak. The symptoms show up as extra noise, exhaust smell, and an engine that behaves differently at idle. Screwing in an O2 sensor plug closes the port and restores a sealed exhaust system.
References & Sources
- JEGS. “O2 Sensor Weld Fittings & Plugs.” Confirms 18mm × 1.5 thread plugs, steel and stainless-steel options, and pricing.
- O’Reilly Auto Parts (Walker). Lists Walker Oxygen Sensor Bung Plug 35299 as a 1-pc threaded bung plug.
- RockAuto (Dorman). Lists Dorman 917112 as an oxygen (O2) sensor plug / bung.