Parts for your 2004 Subaru Impreza-Ignition leads

2004 Subaru Impreza ignition leads — what’s fitted and what to service

Based on Subaru’s 2004 Impreza service manual (Engine Ignition section) and the Subaru FAST parts catalogue, ignition leads are fitted to 2004 Impreza models with the naturally aspirated 2.5‑litre EJ251/EJ253 engines (e.g., GX/RS/TS in various markets). The WRX/STI models with EJ205/EJ207 use coil‑on‑plug ignition and do not have traditional high‑tension leads. Major aftermarket fitment guides (e.g., NGK and equivalent catalogues) mirror this: they list lead sets for the 2.5 NA variants and none for WRX/STI. So, for many 2004 Imprezas, ignition leads are relevant, for WRX/STI, they’re not used because each spark plug has its own coil.

On 2004 Impreza 2.5 NA models, the ignition leads do a deceptively big job. They carry high voltage from the coil pack to each spark plug, reliably and with minimal electrical noise. Quality leads use robust insulation, proper suppression to keep the radio quiet, and the right resistance so the ECU and ignition system stay happy. When they age—heat cycling under the bonnet, oil mist, and general wear—they can crack, arc, or go high‑resistance, which shows up as rough idle, a stumble on take‑off, misfires under load, poorer fuel economy, or hard starting when it’s wet.

Good servicing practice is to inspect or replace the leads alongside spark plugs. A sensible interval is every 60,000–100,000 km or about 5 years, but condition beats the calendar. Check for hardened boots, visible tracking or burn marks, and measure resistance with a multimeter against the spec in the Subaru manual (longer leads will read higher). If one lead is cactus, replace the full set—mixing old and new can mask issues and upset balance.

When fitting, route each lead exactly as per the factory looms and separators, keeping them off hot exhaust components and away from sharp edges. Push boots fully home on the coil towers and spark plugs until they seat with a solid feel, a smear of dielectric grease inside the boots helps sealing and future removal. Don’t yank on the cable—twist the boot to release. While you’re there, inspect the coil pack posts for corrosion and the plug wells for oil, tidy anything suspect before reassembly. Choose reputable OEM‑equivalent suppression leads rather than solid‑core items to keep interference down and the ECU content. Done right, fresh leads restore crisp throttle response and smooth, efficient running on Aussie and Kiwi roads.

• Common symptoms of tired leads: rough idle, hesitation under load, higher fuel use, wet‑weather misfire, visible arcing in the dark.
• Best practice: replace as a full set with new plugs, follow factory routing, and recheck after a short shakedown drive.

Note for WRX/STI owners: your 2004 WRX/STI uses individual coil‑on‑plug units, so there are no ignition leads to replace, focus instead on coil boots and plugs.

Popular questions

Does my 2004 Subaru Impreza actually have ignition leads?
It depends on the variant. The 2.5‑litre naturally aspirated models use a central coil pack with four high‑tension leads. The WRX/STI models run coil‑on‑plug, so there are no traditional leads fitted. A quick look under the bonnet will tell the story—if you see a coil pack with four posts and cables to each cylinder, you’ve got leads.

How often should the leads be replaced on a 2.5‑litre 2004 Impreza?
Plan on 60,000–100,000 km or about five years, or sooner if you notice misfires, cracked insulation, or high resistance on a meter. Heat, oil contamination, and age are the big killers, so condition checks during routine services are well worth it.

What are the tell‑tale signs my leads are on the way out?
Look for rough idle, hesitation on hills, worse fuel economy, or a misfire when it’s damp. You might spot white tracking lines or hear a snapping arc under the bonnet at night. Measuring resistance against the workshop spec is a neat way to confirm suspicion.