P0308 on 2017-2021 Ford F-350 SD: Cylinder 8 Misfire Causes and Fixes

What's Unique About the 2017-2021 Ford F-350 SD

While misfires are common on all vehicles, the 2017-2021 F-350 with the 6.7L Power Stroke diesel has specific, documented issues that can cause a P0308 code. Ford has issued Technical Service Bulletins (TSBs), like TSB 25-2572, pointing to serious internal engine problems such as excessive camshaft and valve tappet (lifter) wear as a potential cause. Forum discussions confirm owners experiencing this failure, sometimes leading to a complete engine replacement. This is a much more severe issue than the typical spark plug or coil failure seen on the 6.2L gas V8.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Rough or shaking idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Flashing or solid Check Engine Light
• Engine may be harder to start
• Smell of unburnt fuel from the exhaust.

Most Likely Causes

1. Faulty Fuel Injector (6.7L Diesel) 🔴 High Probability Diesel injectors operate under extreme pressure and are a common failure point. The Bosch CP4 high-pressure fuel pump used in 2017-2019 trucks can also fail, sending metal debris through the fuel system and destroying the injectors.
   How to confirm: Use a diagnostic scan tool to perform a cylinder balance or contribution test. A failing injector in cylinder 8 will show a clear performance deficit. A physical inspection may also reveal fuel leaks.
   Typical fix: Replace the faulty fuel injector. If the CP4 pump has failed, the entire fuel system (pump, rails, lines, and all 8 injectors) must be replaced. New injectors must be programmed to the PCM.
   Est. part cost: $250-$450 per injector
2. Faulty Ignition Coil (6.2L Gas) 🔴 High Probability Coil-on-plug systems are a very common failure point. Heat and vibration cause the internal windings to fail over time. Corrosion on the coil boots from moisture intrusion into the spark plug wells is also a frequent issue.
   How to confirm: Swap the ignition coil from cylinder 8 with another cylinder (e.g., cylinder 7). Clear the codes and drive the truck. If the code changes to P0307, the coil is the culprit.
   Typical fix: Replace the faulty ignition coil. It's often recommended to replace the spark plug at the same time. Many owners replace all 8 coils as a preventative measure if they are original.
   Est. part cost: $40-$90
3. Worn Spark Plug (6.2L Gas) 🔴 High Probability The 6.2L engine uses two spark plugs per cylinder, an upper and a lower, increasing the number of potential failure points. The recommended service interval is around 60,000 miles, and worn plugs can easily cause a misfire.
   How to confirm: Remove both spark plugs from cylinder 8 and inspect them for wear, fouling, or damage. You can also swap them with another cylinder's plugs to see if the misfire code follows.
   Typical fix: Replace the spark plug(s). It's best practice to replace all 16 spark plugs if they are near the end of their service life.
   Est. part cost: $5-$20 per plug
4. Camshaft and/or Valve Tappet (Lifter) Wear (6.7L Diesel) 🟡 Medium Probability Ford TSB 25-2572 explicitly links P0301-P0308 codes to excess wear on the camshaft and valve tappets in the 6.7L engine. This can be caused by a rocker arm failure that allows the lifter to rotate and be destroyed by the cam lobe. One owner on Ford-Trucks.com reported a P0300 code that was diagnosed as a failed lifter on cylinder #8, which wiped out the cam and required a complete engine replacement.
   How to confirm: This requires significant engine disassembly to visually inspect the camshaft, lifters, and other valvetrain components. Early symptoms can include a ticking noise from the top of the engine. Finding metal shavings in the engine oil during an oil change is a strong indicator of this failure.
   Typical fix: Requires replacement of the camshaft, all lifters, and any other damaged valvetrain components. In severe cases with widespread metal contamination, a complete engine replacement is often necessary.
   Est. part cost: $2000-$5000+

Rare But Worth Checking

• Under-torqued Crankshaft Pulley Bolt: This applies specifically to early 2017 models built on or before February 15, 2017, as noted in Ford SSM 46391. An under-torqued bolt can cause erratic crankshaft position sensor readings, leading to multiple misfire codes including P0308.
• Broken Valve Spring (6.2L Gas): While rare, a broken valve spring can cause a complete loss of compression and a persistent misfire. One owner on a forum with misfires on cylinders 5-8 (P0305, P0306, P0307, P0308) ultimately found a broken valve spring on cylinder #7 to be the cause. This requires removing the valve cover to inspect.
• Low Engine Compression: Caused by internal mechanical issues like worn piston rings, a leaking head gasket, or burnt valves. A compression test is needed to confirm this. This is a serious mechanical issue requiring extensive repairs.
• Damaged Wiring Harness: The wiring or connector for the ignition coil (6.2L) or fuel injector (6.7L) can become damaged from heat, vibration, or rodents, causing an intermittent connection and misfires.
• PCM Software Glitch (6.2L Gas): For 2017-2018 models, TSB 18-2188 notes that misfire codes (P0300-P0308) can occur during cold starts due to spark plug fouling. The fix is a PCM software update to improve cold weather fueling strategies.

Diagnosis Steps

1. Read the code with an OBD-II scanner and check for any other stored codes. Note the freeze frame data.
2. Identify your engine: 6.2L Gas or 6.7L Power Stroke Diesel. The diagnostic path differs significantly.
3. Check for applicable TSBs based on your engine and vehicle build date. TSBs like 25-2572 (6.7L) or 18-2188 (6.2L) can point directly to the cause.
4. For 6.2L Gas: The easiest test is to swap parts. Move the ignition coil from cylinder 8 to cylinder 7. Clear the codes and drive. If the code returns as P0307, you have a bad coil. If not, repeat the process with the spark plugs (both of them).
5. For 6.2L Gas: While the coil is removed, inspect the coil boot and spark plug well for signs of corrosion or moisture, which is a common problem.
6. For 6.7L Diesel: Diagnosis is more complex. Use an advanced scan tool to perform a 'Cylinder Contribution Test' or 'Power Balance Test'. This will show if cylinder 8 is underperforming, strongly suggesting a fuel injector issue.
7. Inspect the wiring and connector for the coil (6.2L) or injector (6.7L) at cylinder 8 for any signs of damage, corrosion, or looseness.
8. For 6.7L Diesel: If the injector seems okay on the power balance test, the issue could be mechanical. Listen for a top-end ticking noise. A compression test may be necessary to check for internal engine problems like the camshaft/lifter wear mentioned in TSB 25-2572.
9. If all else fails, the problem could be a rare wiring issue, a broken valve spring, or a fault in the PCM itself, which may require professional diagnosis.

Parts You'll Likely Need

• Ignition Coil (6.2L Gas) (OEM #Motorcraft DG549) — This is the most common cause of a single-cylinder misfire on the 6.2L gas engine.
  Trusted brands: Motorcraft, NGK, Denso, MSD
  OEM price range: $60-$90
  Aftermarket price range: $40-$70
• Fuel Injector (6.7L Diesel) (OEM #Motorcraft FC4Z-9H529-A (2017-2019) or LC3Z-9H529-A (2020-2022) - verify with VIN) — A primary cause of misfires on the 6.7L Power Stroke, these piezoelectric injectors operate under intense pressure and can fail electrically or mechanically. Cylinders 1, 2, 7, and 8 use a different part than cylinders 3, 4, 5, and 6 on some model years, so it is critical to verify the correct part for cylinder 8.
  Trusted brands: Motorcraft, Bosch, XDP, Bostech
  OEM price range: $350-$500
  Aftermarket price range: $250-$400

Related Codes That Often Appear With This One

• P0300 — If multiple cylinders are misfiring, a P0300 (Random Misfire) code will often accompany the specific cylinder code. This was seen in cases of lifter failure and crankshaft bolt issues.
• P0208 — This code indicates an 'Injector Circuit Malfunction for Cylinder 8' and points directly to an electrical problem with the fuel injector or its wiring, often appearing alongside P0308 on the 6.7L diesel.
• P0358 — For the 6.2L gas engine, this code indicates an 'Ignition Coil H Primary/Secondary Circuit Malfunction', pointing to an electrical issue with the coil or its circuit, rather than just a performance-based misfire.
• P0603, P203B, P2201, etc. — Ford TSBs #21-2248 and #20-2336 list P0300-P0308 along with a large group of other codes on the 6.7L engine. This combination may indicate a need for a Powertrain Control Module (PCM) software update or a more complex electrical issue.

Technical Service Bulletins (TSBs) & Recalls

• TSB 25-2572: Links P0301-P0308 on 6.7L diesel to camshaft and valve tappet (lifter) wear.
• TSB 21-2248 / TSB 20-2336: Lists P0300-P0308 with other codes on 6.7L, may require a PCM update.
• SSM 46391: Notes under-torqued crankshaft pulley bolts on early 2017 models can cause misfire codes.
• TSB 18-2188: Addresses misfire codes on 2017-2018 6.2L gas engines in cold weather, fixed by a PCM update.

Platform-Specific Known Issues

• TSB #25-2572: Some 2021 models may experience a P0308 code due to excessive wear on the camshaft and/or valve tappets, requiring major internal engine repair.
• TSB #21-2248 & TSB #20-2336: On 6.7L engines built before mid-2020/early-2021, a P0308 code may appear with many other DTCs. This could point to a PCM software issue that requires an update from a Ford dealer.
• SSM #46391: On trucks built through February 15, 2017, multiple misfire codes (including P0308) can be caused by an under-torqued crankshaft pulley bolt. This should be checked before beginning other diagnostics on applicable vehicles.

Mechanic-Grade Diagnostic Values

• 6.7L Diesel Fuel Injector Resistance — expected: 150,000 - 210,000 Ohms (150kΩ - 210kΩ). Failure: A reading outside this range, such as 85kΩ or OL (Open Line), indicates a failed injector.
• 6.2L Gas Ignition Coil Primary Resistance — expected: ~0.358 Ohms. Failure: A significant deviation from this value suggests a faulty coil winding.
• 6.7L Diesel Low-Pressure Fuel System — expected: 53-73 PSI. Failure: Pressure below 52 PSI will trigger a 'low' reading on the fuel delivery switch and can starve the high-pressure pump.
• 6.7L Diesel High-Pressure Fuel System (Cranking) — expected: Must meet desired pressure, requires at least 5000 PSI to start.. Failure: If actual pressure does not meet desired pressure while cranking, it points to a high-pressure fuel system issue (pump or leaks).
• Mode $06 Misfire Counts (TID $53, CID $08) — expected: A low number of counts. The threshold for a 'fail' varies with RPM and load.. Failure: A high count for Component ID $08 (representing cylinder 8) confirms the PCM is detecting a significant number of misfire events on that cylinder, even if a code is not yet set.

Hidden / Shadow Codes Worth Checking

• Mode $06, Test ID $53, Component ID $08: This is not a DTC, but a specific data point within the OBD-II system that shows the raw misfire count for cylinder 8 during the current and last driving cycle. It can show misfires that are happening but aren't yet severe enough to trigger a P0308 code. (see via A scan tool that can display Mode $06 data. The value needs to be monitored to see if it increments, indicating an active misfire.)
• P0284 - Cylinder #8 Contribution/Balance: This code is more specific than P0308 and is often set by the PCM after running an automated diagnostic. It directly points to cylinder 8 failing a power contribution test, strongly suggesting a fuel or mechanical issue rather than just an ignition event. (see via This DTC will appear on most standard OBD-II scanners and advanced tools like FORScan or IDS.)

Scan Tool Commands That Help

• Ford IDS / FORScan / High-End Scanners: Power Balance Test (or Cylinder Contribution Test) — This is a critical diagnostic step for the 6.7L Power Stroke. The test graphically displays the performance of each cylinder in real-time. A technician can selectively disable injectors to see the effect on the engine. A cylinder (like #8) with a faulty injector will show little to no change in engine speed when disabled, confirming it wasn't contributing power.
• Ford IDS / FORScan: Injector Quantity Adjustment (IQA) Programming — This is not a diagnostic test, but a mandatory procedure after replacing a fuel injector on the 6.7L Power Stroke. Each new injector has a unique calibration code (IQA code) that must be programmed into the PCM for the cylinder it was installed in. Failure to do so will result in rough running and potential codes.
• FORScan: KOEO Injector Electrical Self Test (Buzz Test) — For diagnosing electrical integrity of the injector circuits. Before starting the engine cold, this test commands all injectors to buzz at once, followed by each injector buzzing sequentially. A weak or absent buzz from cylinder 8 would indicate an electrical issue with the injector solenoid or its wiring.

Wiring & Ground Locations

• G101 (Lower Right Rear of Engine) — Located on the lower right (passenger) side, at the rear of the engine block.. This is a primary engine block ground. A loose or corroded connection here can cause a variety of sensor and actuator issues, including erratic signals for the ignition or injection systems that could lead to misfire detection.
• G102 (Top Rear of Engine - 6.7L) — Located on the top rear of the 6.7L engine.. Another critical engine ground. Given its location, it provides the ground path for many top-of-engine sensors and potentially the injector harness. A poor connection could cause issues specific to the 6.7L Power Stroke.
• Cylinder 8 Injector Connector (6.7L) — On the driver's side (left) valve cover, the rearmost connector.. This is the direct electrical connection to the cylinder 8 fuel injector. It should be inspected for corrosion, damage, or loose pins. The two pins can be probed to measure injector resistance (150kΩ-210kΩ).
• Cylinder 8 Ignition Coil Connector (6.2L) — On top of the rearmost ignition coil on the driver's side (left) valve cover.. This connector provides power and the trigger signal from the PCM to the ignition coil. The wiring harness can become brittle from heat, leading to connection failures that cause a P0308.

Real Owner Repair Stories

• Ford Truck Enthusiasts Forum User (2017 Ford F-350 6.7L Power Stroke) — P0300 (Random Misfire) code, which was later diagnosed to be centered on cylinder 8.
  ❌ Tried (didn't work) Initial diagnosis was difficult due to the random misfire code.
  ✅ What actually fixed it The root cause was a failed lifter on cylinder #8. The failure of the lifter also damaged the camshaft lobe ('wiped out the cam'). The final repair required a complete engine replacement.
• Ford Truck Enthusiasts Forum User (Unknown year, likely V8) — P0308 code.
  ❌ Tried (didn't work) Did not change the spark plug initially due to fear of breakage.
  ✅ What actually fixed it Replacing the Coil-On-Plug (COP) for cylinder 8 resolved the P0308 code. However, the user later experienced sequential misfires on other cylinders (P0307, P0306), suggesting a series of age-related coil failures.

OEM Part Supersession History

• FC3Z-9H529-A / FC3Z-9H529-B → FC4Z-9H529-A — Updated design for 2017-2019 6.7L Power Stroke fuel injectors.
  Heads up: Injectors for 2017-2019 models are different from 2020+ models due to a fuel system redesign. Furthermore, on 2015-2019 trucks, injectors for cylinders 1, 2, 7, 8 are a different part than for cylinders 3, 4, 5, 6. It is critical to verify the correct part for cylinder 8 for the specific model year.

Model Year Variations Within This Range

• 2020-2021: The 6.7L Power Stroke engine received a significant update for the 2020 model year. This included a new higher-pressure fuel injection system running at 36,000 PSI (up from ~30,000 PSI), new injectors, and an upgraded fuel pump. This means fuel system components, especially injectors like the one for cylinder 8, are not interchangeable between 2017-2019 and 2020-2021 models.
• 2017 (Early Production): As per Ford SSM 46391, vehicles built on or before February 15, 2017, may have an under-torqued crankshaft pulley bolt. This specific issue can cause multiple misfire codes, including P0308, due to erratic crankshaft position sensor readings. This is not a concern for later models.
• 2017-2019: These model years use the Bosch CP4 high-pressure fuel pump, which is known for a failure mode that can send metal debris throughout the fuel system, destroying all injectors. While not a direct cause of a single injector misfire, a P0308 could be the first symptom of this catastrophic failure beginning.