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Mitsubishi Lancer, Galant, and Eclipse Oxygen Sensor Replacement Guide is a practical reference for owners, technicians, and buyers comparing replacement parts. It explains what to verify before purchase, how to recognize common symptoms, and how to plan a safe installation without relying on assumptions about year, trim, or engine configuration.

Oxygen sensor failures on these Mitsubishi models commonly trigger diagnostic trouble codes such as P0135, P0141, P0155, and P0157. P0135 and P0155 point to heater circuit malfunctions in the upstream sensors, while P0141 and P0161 point to downstream sensor heater issues. If your scanner shows any of these codes, work through the checks below before you order anything.

How the oxygen sensor system works

Mitsubishi Lancer, Galant, and Eclipse engines — including the 4G63 and 4G64 variants — use oxygen sensors in two positions, and the two positions do two different jobs. Replacement should always use the correct OEM or equivalent sensor for your engine family. A sensor for the wrong family or the wrong position will not clear the fault.

Upstream sensor: before the catalytic converter

The upstream sensor sits before the catalytic converter and monitors air-fuel ratio. When its heater circuit malfunctions, the scanner typically shows P0135 or P0155. Treat those two codes as an upstream-heater signal first, and verify that sensor before you look anywhere else.

Downstream sensor: after the catalytic converter

The downstream sensor sits after the catalytic converter and monitors catalyst efficiency. Heater faults on this side typically log as P0141 or P0161. Front and rear sensors do different jobs, so they are not interchangeable even when the bodies look alike.

What to verify before ordering

Start with the vehicle year, make, model, engine or drivetrain, and the original equipment number printed on the removed part. Match connector shape, mounting points, dimensions, and left/right position where applicable. A similar-looking component can still be incompatible, so confirm the seller's fitment notes and compare the original part before installation.

• Record the VIN and exact trim level.
• Compare the OE or interchange number and connector layout.
• Check whether the part is front/rear, left/right, or a complete set.
• Inspect nearby components that could cause the same symptom.

Replacement options that cover these models

For the widest coverage window, compare your OE number against the Oxygen Sensor compatible with Mitsubishi 1989-2017 Lancer Eclipse Galant. That listing spans the 1989-2017 range across the three models, which makes it the first place to check for older builds.

If your car falls in a tighter window, the Oxygen Sensor compatible with Mitsubishi 1992-2012 Eclipse Galant Lancer covers the 1992-2012 span. Either way, the VIN and the OE number on the removed sensor decide the match — not the product photo.

Why a visual match is not enough

Connector layouts, wire lengths, and thread details differ between positions and model years. Two sensors can look nearly identical and still fail to plug in or seat correctly. Hold the new sensor next to the old one before you thread anything into the exhaust.

Installation and diagnostic checklist

An oxygen sensor threads into the exhaust, so heat is the first safety issue. Always disconnect the battery and allow the exhaust to cool before you start. Support the vehicle correctly and keep the workshop manual open for torque values and calibration steps.

While you work

Clean the mating surfaces before installing the new sensor, and apply anti-seize compound only if specified. Route the wiring away from heat and moving parts — a melted or chafed harness will bring the same heater-circuit code back within days.

Before you call the job done

Test the repair before returning the vehicle to service. If a warning light remains on, scan the relevant diagnostic codes and verify the repair instead of replacing additional parts at random. Recheck power, grounds, fasteners, and alignment, then compare the installed part with the removed component.

Specification summary

How to choose the right replacement

Prioritize verified compatibility, material quality, warranty coverage, and clear product photos. For safety-critical systems such as brakes, steering, sensors, and fuel delivery, use the vehicle-specific specification rather than a universal substitute. Keep the original part until the new component has been test-fitted and confirmed.

One nearby component worth checking

The ordering checklist tells you to inspect nearby components that could cause the same symptom, and on the intake side that means the air meter. KOEEP carries the MAF Sensor Meter MD357335 For Mitsubishi Lancer GALANT L 400 SPACE RUNNER WAGON for exactly these Lancer and Galant applications.

Electrical parts and further reading

Heater-circuit codes are electrical faults as often as they are sensor faults, so it pays to look at the wiring and ECU side of the car. The Starters, Alternators & ECUs | Koeep Auto & Moto Electrical Parts collection gathers starters, alternators, ECUs, and wiring parts in one place.

If you also maintain an Evolution, the Complete Guide to Alternator for Mitsubishi Lancer Evolution Final Edition 2015 covers the charging system in the same direct, practical style.

Planning a bigger service? Two KOEEP guides cover the jobs that usually come due around the same mileage:

• The complete timing belt and water pump kit guide for the 2004-2009 Mitsubishi Lancer and Eclipse 2.4L walks through that essential maintenance.
• The MD305600 MAP / MDP sensor guide explains OEM compliance for EVO 7/8, 2G DSM Eclipse, and Talon owners.

Frequently asked questions