Parts for your 2003 Subaru Impreza-Ignition coils

2003 Subaru Impreza ignition coils — what they do and when to replace

Ignition coils are absolutely relevant and fitted to the 2003 Subaru Impreza. Technical documentation confirms this: the Subaru Factory Service Manual for MY03 Impreza details individual coil-on-plug units on the EJ205 WRX engine and a four-tower wasted-spark coil pack with leads on non‑turbo EJ20/EJ25 engines (Subaru STIS Workshop Manual, Engine section). This layout is also reflected in mainstream references like the Haynes Repair Manual for Subaru Impreza 2002–2008 and application catalogues from OE suppliers such as Denso and NGK.

The job of the ignition coil is to step 12 volts up to tens of thousands of volts to fire the spark plugs right on cue. When a coil gets tired, the Impreza can show a rough idle, flat spots under load, higher fuel use, and a glowing Check Engine light with misfire codes (P0301–P0304). On the WRX, a single failing coil will usually misfire on that one cylinder, on non‑turbo models, a failing coil pack can take out a pair of cylinders because it fires in wasted‑spark pairs.

• Common checks: inspect connectors and earths, look for cracked boots (COP), test plug leads for resistance (wasted‑spark), and follow the workshop manual test procedure for coil resistance and power supply.
• Good practice: use quality iridium/platinum plugs, keep moisture and oil out of the plug wells, and use a dab of dielectric grease on COP boots.

Coils aren’t a scheduled replacement item, but they should be inspected whenever spark plugs are changed—typically around 100,000 km for long‑life plugs, or earlier on turbo engines that see spirited driving. If replacing, stick with OE‑quality units (Denso/NGK) and, on wasted‑spark models, consider renewing the plug leads at the same time. Always disconnect the battery, label connectors, and torque fasteners and spark plugs to workshop specs to avoid cracked ceramics or damaged threads under the bonnet.

For WRX EJ205 owners: replace the affected coil-on-plug, check the matching plug condition, and clear codes after repair. For non‑turbo EJ20/EJ25 owners: a new coil pack and fresh leads can restore a silky idle and crisp throttle response in one go. Following the factory procedure (Subaru STIS/FSM) keeps things tidy and avoids repeat faults.

• What type of ignition coil does a 2003 Impreza have?
  The 2003 WRX (EJ205) uses four individual coil‑on‑plug units—one per cylinder. Most non‑turbo models run a single four‑tower coil pack with high‑tension leads to the plugs. This split is documented in the Subaru workshop manual and mirrored by OE parts catalogues.
• How often should the coils be replaced?
  They’re not a routine service item. Inspect them at each spark plug change (around 100,000 km) or sooner if you notice misfires, rough running, or fault codes. Replace only when testing points to a fault, and use OE‑quality parts.
• Can a bad coil harm the catalytic converter?
  Yes. A persistent misfire can send unburnt fuel down the exhaust, overheating and damaging the cat. If the car is missing under load, ease off, avoid long drives, and sort the coil or related ignition issue promptly.