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P0309 only sets on V10 or V12 engines. If you see this code on a V8, the ECM has gone haywire. Common candidates: Ford Excursion / E-350 / F-250 with the 6.8L V10 Triton, BMW 7-series V12, Mercedes S600 V12, Audi S8 W12. Start with plug and coil swap to cylinder 10. See top-rated scanners on Amazon ↑
🗺️ Where Is the Problem?
Blueprint view - P0309 fault localized to cylinder #9 (spark plug, ignition coil, or fuel injector)
These are statistical causes across ALL vehicles - your exact car may rank differently
For example, on a Honda 4-cyl the downstream O2 sensor causes P0309 64% of the time, but on a GM 5.3L V8 the catalytic converter is the cause 71% of the time. Get a probability ranking built specifically for your year, make, model, and mileage.
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🎯 Top Causes & Probability
55%
#1 - Most Likely
Failed Coil-on-Plug (Cylinder 9)
On the Ford 6.8L V10 Triton, coil-on-plug failures are extremely common at 80k+ miles. The boot cracks and arcs to the head. On BMW V12 and Mercedes V12, coils on the rear bank are heat-stressed. Replace with OEM - aftermarket coils on these engines fail fast.
🔩 Part
$30–$120
👨🔧 Labor
$50–$150
⚡ DIY Difficulty
Easy
30%
#2 - Check First
Worn or Fouled Spark Plug (Cylinder 9)
Plugs on V10/V12 engines reach the back of a long head and rarely get serviced on schedule. A worn or oil-fouled plug on cylinder 9 can't fire reliably. Note: Ford Triton plugs are notorious for snapping off in the head - soak with penetrant before removal.
🔩 Part
$8–$25
👨🔧 Labor
$30–$120
⚡ DIY Difficulty
Medium
15%
#3 - Less Common
Clogged or Failed Injector (Cylinder 9)
On V10/V12 engines, rear-bank injectors get less throttle blip and tend to clog. Severe carbon buildup or a stuck injector causes a lean misfire. A noid light or scope test on the connector reveals if the PCM is firing the injector correctly.
🔩 Part
$80–$300
👨🔧 Labor
$100–$200
⚡ DIY Difficulty
Hard
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Locate cylinder 9 - on Ford V10 Triton it's the 4th cylinder on the passenger bank. On BMW/Mercedes V12 it's usually rear-most on one bank. Check your service manual.
- Swap coil cyl 9 ↔ cyl 10 - clear codes, drive 10 minutes. If P0310 sets, coil is bad. Cheapest possible diagnosis.
- Swap plug cyl 9 ↔ cyl 10 - if coil swap didn't move it, plug is suspect. On Triton V10s, use anti-seize sparingly and torque to spec to avoid blow-out.
- Compression test cylinder 9 - rule out mechanical failure. V10/V12 engines with high miles can drop a valve seat.
- Check injector resistance and waveform - final step before assuming worn-out engine. Scope on the injector control wire shows if PCM is firing it correctly.
📍 Find a Trusted Shop Near You
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Tips for Choosing a Shop
- Ask if they charge a diagnostic fee and whether it applies toward the repair
- Request a written estimate before approving any work
- Ask specifically about the part brand - OEM vs. aftermarket matters for this code
- Check Google reviews for recent mentions of the specific repair you need