P0304 on Saab: How to Diagnose & Fix
Cylinder 4 Misfire Detected
2 Saab models · 2 documented issues · 2 critical · $80-$600 repair
P0304 on Saab by model: symptoms, diagnosis and fix
The Direct Ignition Cassette, which sits directly on top of the spark plugs and combines the coils into one unit, is a frequent failure point on the four-cylinder cars. Underhood heat and valve-cover gasket oil leaks contaminating the coil pack degrade the cassette over time, causing misfires, rough idle, hard starting and stalling. A failed cassette typically has a burnt smell and discoloration around the plug boots.
Common Symptoms
- Engine misfire
- Rough idle
- Hard starting / crank no-start
- Stalling while driving
- Check engine light
- Burnt smell from engine bay
How to Fix
Replace the Direct Ignition Cassette (use a quality OEM-grade unit) and renew the spark plugs at the same time, using the correct Saab-specified plugs and dielectric grease. Fixing any valve-cover gasket leak prevents repeat failures.
Sources for this issue (3)
The Saab Direct Ignition Cassette — single integrated unit covering all four spark plugs — is a well-known wear item on the first-generation 9-3 (1999-2003, B204/B205/B235 engines) and the 9-5; the 2003+ second-generation 9-3 with the B207 uses separate ignition coils instead. Symptoms: misfire (often cylinder-specific then random), reduced power, check engine light. Failure rate climbs sharply after 80,000 miles. Saab DIC heat-shielding cracks; rubber boots fail.
Common Symptoms
- misfire
- reduced power
- check engine light
- no-start
- rough idle
How to Fix
Replace DIC ($150-$350 OEM cassette, $80-$180 aftermarket — quality matters here, cheap units fail in months). DIY ~30 minutes with a 10mm hex socket. Always replace spark plugs at the same time with Saab-specified resistor plugs (NGK PFR6H-10 on the B205, B235 and 1999 200 hp B204; NGK BCPR7ES on the 1999 185 hp B204). Carry a spare DIC if traveling long distances — failure is sudden but the car limps home on the remaining cylinders.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Saab Genuine Trionic 7 black direct ignition cassette 32022255 (previous 55559955)
Not B207 Sport Sedan and not red Trionic 5 cassette cars; confirm VIN/engine.
- NGK PFR6H-10 spark plug, Saab 9399866
Confirm engine, tune/heat-range requirements and gap.
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Sources for this issue (2)
P0304 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0304 mean on a Saab?
P0304 stands for "Cylinder 4 Misfire Detected." This code means the engine computer detected a misfire in cylinder number 4 — that cylinder isn't igniting its air-fuel mixture fully or at all. The system detects this by monitoring small changes in crankshaft rotation speed as each cylinder fires. You may feel a rough idle, shaking, hesitation or reduced power, and a persistent misfire can dump unburned fuel into the exhaust and overheat or damage the catalytic converter. Since it's pinpointed to one cylinder, the problem typically lies in that cylinder's spark, fuel, or compression. On Saab it is documented across 2 models: 9-5, 9-3.
What are the most common causes of P0304?
The most common causes of P0304 are: Worn or fouled spark plug in cylinder 4, Failed or weak ignition coil on cylinder 4, Faulty spark plug wire or boot (where applicable), Clogged, stuck or leaking fuel injector for cylinder 4, Vacuum or intake air leak near that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged wiring or connector at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0304 on a Saab?
Published repair estimates on Saab range from $80 to $600 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.