Oxygen Sensor 8M2003959 – Fits MerCruiser EC Sterndrive

Vendor: JLM Marine
Availability: In Stock
Product Type: Sensor
$96.99
$96.99
Subtotal: $96.99

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Oxygen Sensor 8M2003959 – Fits MerCruiser EC Sterndrive

Oxygen Sensor 8M2003959 – Fits MerCruiser EC Sterndrive

$96.99

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. 1 Stop the engine and let the exhaust system cool completely, then disconnect the battery negative cable before touching any sensor wiring.
  2. 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. 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. 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. 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. 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

How do I confirm this fits my MerCruiser? +

Start with your engine serial number, then match the model and EC designation in the compatibility table, and finally compare the number on your original sensor with 8M2003959 or 710-8M2003959. The connector and lead form should also match the product images.

Is this for EC engines only? +

Yes. Quicksilver documents 8M2003959 for MerCruiser EC sterndrive applications. Engines built without a catalyst / EC exhaust system do not use this sensor position.

What are common symptoms of a failing oxygen sensor? +

Higher fuel use, rich or rough idle, hesitation under load, and stored fuel-system fault codes on a SmartCraft display or diagnostic tool. A rich mixture can also foul spark plugs, so inspect plugs when replacing the sensor.

Is this an OEM or aftermarket part? +

This is an aftermarket replacement manufactured by JLM Marine. It is not a genuine Mercury or Quicksilver part. OEM numbers are listed only for cross-matching.

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.

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