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Oxygen Sensor 8M2003959 – Fits MerCruiser EC Sterndrive
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How to Match This Part
This aftermarket exhaust oxygen (O2) sensor replaces Mercury MerCruiser / Quicksilver 8M2003959 (also catalogued as 710-8M2003959). It threads into the exhaust path on selected MerCruiser EC (emissions-controlled) sterndrive and inboard engines and reports exhaust oxygen so the ECM can hold closed-loop air/fuel control. A failed sensor commonly causes rich running, poor fuel economy, and stored fuel-system codes.
Match this part in this order:
1. Engine serial number — MerCruiser sterndrive and inboard: engine block or flame-arrestor cover plate
2. Engine model and EC designation — confirm your row in the compatibility table below
3. OEM number on the removed sensor — 8M2003959 or 710-8M2003959
Physically this listing is one sensor: threaded probe with perforated tip shield, hex flats for an oxygen-sensor socket, black corrugated lead sleeve, and a sealed four-pin connector with a blue weather seal and locking tab. Compare the connector and lead to the part you remove, and see the product images for the exact form.
Part Number Cross-Reference
Mercury publishes this sensor as 8M2003959. The same Quicksilver / MerCruiser part also appears under the catalog form 710-8M2003959. Either number on your removed sensor or parts list points to this application.
| Type | Brand | Part Number | Notes |
|---|---|---|---|
| OEM | Mercury MerCruiser / Quicksilver | 8M2003959 | Primary reference (SENSOR O2) |
| OEM | Mercury MerCruiser / Quicksilver | 710-8M2003959 | Same part — Quicksilver catalog form |
Part numbers are listed for identification only. This is an aftermarket part manufactured by JLM Marine and is not sold as a genuine Mercury or Quicksilver component.
Compatible Engine Models
Confirm model and serial before ordering. Mercury lists this sensor for MerCruiser EC applications by serial break, not by model year. MerCruiser serial location guide: Mercury Parts Diagrams. Engines without an EC / catalyst exhaust system do not use this sensor position.
| Engine Model | Drive / Configuration | Serial Number Range |
|---|---|---|
| 3.0L MPI EC | Sterndrive | 1A072812 & Up |
| 4.3L MPI Alpha / Bravo EC | Sterndrive | 1A637496 & Up |
| 4.5L MPI 200 / 250 Alpha / Bravo (incl. EC) | Sterndrive | 2A441953 & Up |
| 5.0L MPI Alpha / Bravo EC | Sterndrive | 1A300000 & Up |
| 350 MAG MPI Alpha / Bravo EC / Horizon EC | Sterndrive | 1A300000 & Up |
| 377 MAG MPI Bravo EC | Sterndrive | 1A343300 & Up |
| 496 MAG EC (Base / H.O.) | Sterndrive | 1A066166 thru 1A350440 |
| 5.7L MPI MIE EC / 5.7L SKI EC | Inboard / Ski | 1A090000 & Up |
| 6.2L MPI MIE EC | Inboard | 1A091600 & Up |
| 8.1L Inboard EC (Base / H.O.) | Inboard | 0W697154 thru 1A380251 |
| 8.2L Inboard EC / 8.2L Mag EC (Base / H.O.) | Inboard / Sterndrive | 1A350340 & Up (confirm break for your build) |
| Scorpion 350 EC | Sterndrive | 0W698433 & Up |
| Scorpion 377 HO EC | Sterndrive | 1A091488 thru 1A092170 |
| 520 / 540 | Race / performance | 0M970122 & Up / 0M970866 & Up |
| 100 Vazer EC | Sterndrive | 1A035295 thru 1A036006 |
Technical Specifications
| Specification | Value |
|---|---|
| Part type | Exhaust oxygen (O2) sensor for MerCruiser EC closed-loop fuel control |
| Mounting | Threaded into the exhaust component boss |
| Probe form | Metal tip with perforated protective shield; hex flats for sensor socket |
| Connector | Sealed 4-pin plug with blue weather seal and locking tab |
| Harness | Black corrugated protective sleeve over the wire leads |
| Application class | MerCruiser EC (emissions-controlled) sterndrive and selected inboard engines |
Installation Guide
- 1 Stop the engine and let the exhaust system cool completely, then disconnect the battery negative cable before touching any sensor wiring.
- 2 Trace the harness from the exhaust to the engine harness, release the locking tab and unplug the four-pin connector, then free the leads from any clips or ties.
- 3 Unscrew the old sensor with an oxygen-sensor socket or an open-end wrench that clears the harness. Inspect the threads in the exhaust boss and clean them if needed.
- 4 Compare the new sensor against the old one for thread form, hex size, harness routing, and connector shape. Keep oil, sealant, and solvents off the sensing tip.
- 5 Thread the sensor in by hand, then tighten it to the torque value in your MerCruiser service manual. Route the harness away from hot exhaust surfaces and moving belts, and refit the original clips.
- 6 Reconnect the connector until the lock clicks, reconnect the battery, clear any stored fault codes with a marine diagnostic tool, and run the engine to confirm the fault does not return.
Note: Professional installation is recommended for exhaust and closed-loop fuel-control components.
Troubleshooting & FAQ
Important Notes
- Serial number breaks matter. Confirm your engine serial is inside the listed range before ordering.
- This sensor serves MerCruiser EC applications. Non-EC engines need a different sensor family.
- Do not confuse this MerCruiser EC sensor with Mercury outboard oxygen sensors that use different OEM numbers.
- Professional installation is recommended for exhaust and closed-loop fuel-control components.
Why Choose JLM MARINE?
- Precision Fit: Cross-referenced to Mercury MerCruiser / Quicksilver 8M2003959 for selected EC sterndrive and inboard applications.
- Quality Control: Each batch is inspected for dimensional consistency and workmanship.
- Direct from Manufacturer: Aftermarket marine parts engineered for reliable exhaust oxygen sensing on MerCruiser EC fuel systems.
We can ship to virtually any address in the world with free shipping available for most regions.
For our typical US and Canadian customers , our shipping timeframes are as follows:
- Amazon FBA: orders will be delivered within 1-5 business days after placing the order (during certain special periods such as major promotions, Christmas, etc., the delivery time of Amazon FBA may be affected).
- US warehouse shipment: orders will be delivered within 5 days after placing the order.
- China warehouse shipment: orders will be delivered about 12 days after placing the order.
When you place an order, we will estimate shipping and delivery dates for you based on the availability of your items and the shipping options you choose. Depending on the shipping provider you choose, shipping date estimates may appear on the shipping quotes page.
Please also note that the shipping rates for many items we sell are weight-based. The weight of any such item can be found on its detail page. To reflect the policies of the shipping companies we use, all weights will be rounded up to the next full pound.
Founded in 2002, JLM Marine has established itself as a dedicated manufacturer of high-quality marine parts, based in China. Our commitment to excellence in manufacturing has earned us the trust of top marine brands globally.
As a direct supplier, we bypass intermediaries, which allows us to offer competitive prices without compromising on quality. This approach not only supports cost-efficiency but also ensures that our customers receive the best value directly from the source.
We are excited to expand our reach through retail channels, bringing our expertise and commitment to quality directly to boat owners and enthusiasts worldwide.
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