Oxygen Sensors for Subaru Impreza (1999-2004) 2.5L Downstream

Direct-Fit Downstream Oxygen Sensor for 1999-2004 Subaru Impreza

What it Does: The Downstream Oxygen Sensor monitors post-catalytic exhaust stream chemistry on the 1999-2004 Subaru Impreza to ensure proper catalyst efficiency, maintain closed-loop trim balance, and eliminate false emissions trouble codes.

The Stock Subaru Impreza Sensor Fouling and Thermal Degradation

Factory oxygen sensors on the EJ251 naturally aspirated boxer platform endure brutal thermal cycling due to their proximity to the catalytic converter under the transmission tunnel. Over time, original zirconia sensing elements suffer from soot accumulation, silicone poisoning, and thermal fatigue. This degradation leads to sluggish response rates and incorrect downstream voltage signals, frequently triggering intermittent P0420 catalytic efficiency diagnostic trouble codes and degraded closed-loop secondary fuel corrections.

The Engineering Solution

This replacement downstream sensor utilizes a fast-reacting planar ceramic sensing cell enclosed in a high-grade 304 stainless steel housing. Engineered with an integrated high-efficiency heating circuit, it reaches optimal light-off operating temperatures rapidly to prevent cold-start enrichment errors. Precision-machined M18x1.5 threads ensure gall-free installation into the exhaust bung, while a heavy-duty submersible harness with an OEM-spec weather-pack connector resists heat exposure and moisture intrusion under aggressive chassis vibration.

Technical Specifications

Compatibility

• 1999-2001 Subaru Impreza 2.5RS Coupe/Sedan (GC6) 2.5L H4
• 2002-2004 Subaru Impreza 2.5RS Sedan (GD6) 2.5L H4
• 2002-2004 Subaru Impreza 2.5TS Sport Wagon (GG6) 2.5L H4
• 2002-2004 Subaru Impreza Outback Sport Wagon (GG6) 2.5L H4

Frequently Asked Questions

Does this fit the GC and GD Impreza?
Yes, this is a direct fit for the 1999-2004 Subaru Impreza equipped with the 2.5L EJ251 engine in the downstream position.

Will this sensor clear a persistent P0420 Check Engine Light?
If your P0420 code is triggered by a biased, slow-responding downstream sensor rather than an exhaust leak or failed converter core, this direct replacement will restore proper diagnostic voltage switching.

The Subaru Performance Caliber

Proper sensor feedback is critical to maintaining crisp boxer engine throttle response and stable fuel trims. Built to survive rally stages and daily commuting alike, this sensor restores OEM diagnostic accuracy without compromise.