P0308 on Audi: How to Diagnose & Fix
Cylinder 8 Misfire Detected
4 Audi models · 4 documented issues · $350-$1,800 repair
P0308 on Audi by model: symptoms, diagnosis and fix
All A8 models with direct-injection engines (3.0T, 4.0T, 60 TFSI) suffer from severe carbon buildup on intake valves. In direct-injection engines, fuel bypasses the intake valves, leaving them exposed only to oil vapors from the PCV system. These vapors bake onto valve backs as hard carbon deposits over 60,000-100,000 miles. Carbon restricts airflow, causing rough idle, misfires, hesitation, and power loss. The high-performance engines in the A8 are particularly susceptible due to higher oil vapor pressure. The ONLY effective fix is walnut blasting every 60,000 miles. This is preventive maintenance, not optional. Failure to clean carbon can cause valve damage requiring engine replacement on the A8's expensive V6/V8 engines.
Common Symptoms
- Rough or unstable idle
- Hesitation or stumbling on acceleration
- Noticeable power loss
- Poor fuel economy (3-5 MPG drop)
- Check engine light with misfire codes (P0300-P0308)
- Engine runs rough when cold
- Hard starting or extended cranking
How to Fix
WALNUT BLASTING: Remove intake manifold and blast crushed walnut shells through intake ports ($1,000-$1,800 for A8 due to packaging complexity). Requires specialized equipment. Repeat every 60,000 miles as PREVENTIVE maintenance. PREVENTION: Install catch can ($400-$700) to filter PCV vapors—extends cleaning interval to 100k+ miles. Add Liqui Moly Intake Valve Cleaner to every oil change. Change oil every 5,000 miles to reduce vapor contamination. Budget for this as routine maintenance.
Sources for this issue (1)
Audi guidance documents that fuel-related deposits can cause a rough cold start, hesitation, reduced performance and misfire faults. For the B7 RS4 4.2L FSI V8, Audi TSB 2022030/2 treats P0300 and cylinder-misfire faults as a diagnostic starting point and first distinguishes injector deposits from other causes. These symptoms do not by themselves prove intake-valve buildup, and Audi does not specify a recurring 30,000-60,000-mile cleaning interval in the cited guidance.
Common Symptoms
- Rough running after a cold start
- Cold-start misfire with the malfunction indicator lamp on
- Hesitation or reduced engine performance
How to Fix
Have the cold-start event and stored faults diagnosed before authorizing intake cleaning. The B7 RS4 bulletin first checks whether its fuel-additive and follow-up procedure resolves injector-deposit misfires. Current TSB 2014753/13 warns that gasoline additive can clean FSI injectors and combustion chambers but not FSI intake valves. If the misfire remains, the shop should continue guided diagnosis and confirm inlet-valve deposits before using the applicable mechanical-cleaning procedure. Do not buy a scanner, oil bundle or cleaning service solely from this card.
Sources for this issue (2)
The coil-on-plug ignition coils on both the 4.2 V8 and 5.2 V10 are a recurring weak point, often failing between 50,000 and 80,000 miles. A failing coil produces a weak or inconsistent spark, triggering cylinder misfires, rough idle, hesitation and a check-engine light. Owners frequently report that oil weeping from valve-cover gaskets or spark-plug tube seals collects in the plug wells and accelerates coil breakdown by overheating and shorting the coil. Because the V8 has 8 coils and the V10 has 10, chasing one bad coil at a time is common but shops recommend replacing them as a set.
Common Symptoms
- Rough idle and vibration at a stop
- Hesitation or power loss on acceleration
- Flashing or steady check-engine light
- Cylinder-specific misfire codes
- Increased fuel consumption
How to Fix
Diagnose the misfiring cylinder(s) via the stored misfire counters, then replace the failed coil(s) along with spark plugs; most specialists replace the full set of 8 (V8) or 10 (V10) plus plugs to avoid repeat visits. The canonical 4.2 V8 coil is the revision-'T' red coil, Audi/VW PN 077905115T (superseded family, aftermarket cross-reference 06E905115E). The Gen-2 5.2 V10 uses Audi PN 07L905115B. Address any valve-cover or plug-tube oil leak at the same time so new coils are not contaminated.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Ignition CoilCross-ref0221604800
Bosch Engine Management Sensors 0221604800. Fitment confirmed against the ShowMeTheParts catalog for this vehicle's year and engine.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (1)
The eight individual coil-on-plug ignition coils on the 4.0T V8 are a recurring misfire source as they age, especially on tuned cars where higher cylinder pressures stress the spark. A failing coil produces a cylinder-specific misfire (P030X) and a flashing or steady check engine light. Owners and shops commonly diagnose by swapping a suspected coil to a different cylinder — if the misfire code follows the coil, the coil is confirmed bad. This is distinct from the already-documented fuel-injector deposit misfires and carbon-buildup issues: here the root cause is the ignition coil itself (or its companion spark plug), not fueling.
Common Symptoms
- Cylinder-specific misfire / rough idle
- Flashing or steady check engine light
- Hesitation or stumble under acceleration
- Misfire code that moves when the coil is swapped to another cylinder
- Reduced power and rough running, worse when cold or under boost
How to Fix
Confirm the failing cylinder, then swap the suspect coil to another cylinder to verify the fault follows the coil. Replace the failed coil; best practice on the 4.0T is to replace all eight coils and the spark plugs as a set since the access labor is shared and mixed-age coils tend to fail in sequence. Use OE or quality OE-equivalent coils (the 4.0T uses the 079905110-series coil). Inspect spark plugs for fouling/oil that could indicate a secondary issue.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Ignition CoilCross-ref0986221138
Bosch Engine Management Sensors 0986221138. Fitment confirmed against the ShowMeTheParts catalog for this vehicle's year and engine.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (1)
P0308 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 P0308 mean on a Audi?
P0308 stands for "Cylinder 8 Misfire Detected." P0308 means the engine computer detected a misfire on cylinder number 8 — that cylinder is failing to combust properly or at the correct time. The PCM identifies it by monitoring crankshaft speed variations as cylinder 8 fires. Misfires waste fuel, increase emissions, and can overheat and damage the catalytic converter, so a hard misfire may cause a flashing check engine light. Typical symptoms include rough idle, vibration, hesitation, and reduced power. On Audi it is documented across 4 models: A8, RS4, R8, S7.
What are the most common causes of P0308?
The most common causes of P0308 are: Worn, fouled, or incorrectly gapped spark plug on cylinder 8, Failed ignition coil or coil-on-plug for cylinder 8, Faulty, clogged, or leaking fuel injector, Vacuum or intake air leak affecting that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged spark plug wire or boot (on applicable engines), Carbon buildup or sticking valve. The specific cause varies by vehicle.
How much does it cost to fix P0308 on a Audi?
Published repair estimates on Audi range from $350 to $1,800 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.
Looking for P0308 on a different make?
View P0308 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.