OBD-II trouble code
P0284: Cylinder 8 Contribution/Balance Fault
Cylinder 8 is at the end of its bank on every V8 — end of the fuel rail, end of the injector harness, last in line for everything. Before you condemn the cylinder, check whether it is simply the first place a marginal supply shows up.
Quick facts
- System
- Powertrain
- Category
- Fuel Injectors
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $3,500
- DIY difficulty
- Intermediate DIY
Browse every code in P0200–P0299, or start from the full code library.
What does P0284 mean?
The module watches crankshaft speed at high resolution and measures the small acceleration each power stroke produces. Comparing those accelerations across cylinders tells it which ones are pulling their weight. This code says cylinder 8 is not, and it is a statement about output — not about a wire, and not about a specific component.
What is worth adding on this particular cylinder is where it physically sits, because on a V8 that position is not neutral.
Cylinder 8 is the last cylinder on its bank. On a longitudinal V8 that means the rearmost position, hard against the bulkhead. Three consequences follow, and none of them apply to a cylinder in the middle of an engine.
The first is fuel. Cylinder 8's injector is at the far end of the rail's run. Rail pressure is not perfectly uniform along that length under demand, and when supply is marginal — a tiring pump, a restricted filter, a partly blocked strainer — the deficit is most visible at the most distant tap. A single weak cylinder at the end of the rail with the rest of the engine healthy is therefore worth one supply-side check before it is treated as a cylinder problem at all. Measure pressure under load and see whether it holds.
The second is electrical. Cylinder 8 is also at the end of the injector harness branch, which means the longest conductor run and the most connectors between the module's driver and the solenoid. Voltage drop accumulates along that path. An injector at the end of the branch will open marginally later than one near the front if the harness has developed resistance, and a slightly late injector delivers slightly less fuel — which is exactly what a balance monitor reports. Measuring supply voltage at the cylinder 8 injector connector, rather than at the rail's feed point, takes minutes and occasionally ends the diagnosis.
The third is money. On nearly every V8 the rearmost injector is under the upper intake plenum, and reaching it means gaskets and hours. That inverts the usual economics of guessing: on an accessible cylinder, fitting an injector to find out is a defensible shortcut, but here the teardown can cost more than the part. Compression and leak-down testing that cylinder first is the single highest-value thing you can do, because a worn ring pack or a burnt valve produces an identical reading at the crankshaft and no injector will fix it.
One trap specific to a crank-based monitor. The module infers which cylinder was weak from when in the rotation the deficit appeared. If the crankshaft position signal is degraded, or the reluctor wheel is damaged or has shifted, attribution can land on a cylinder adjacent in the firing order rather than on the real offender. Before committing to a teardown, look up the engine's firing order, identify which cylinders fire immediately before and after cylinder 8, and check whether their contribution figures also look marginal. A clean single-cylinder deviation is trustworthy; a smeared one across firing-order neighbours is a reason to look at the crank sensor signal first.
A note on identification that catches people constantly: manufacturers do not agree on where cylinder 8 is. General Motors V8s number the passenger bank 2-4-6-8. Ford modular V8s number the driver's bank 5-6-7-8. Same code, opposite sides of the engine. Use the engine's own cylinder diagram, never the firing order, which tells you when a cylinder fires and not where it lives.
Most real-world traffic on this code comes from diesel V8s — the Power Stroke and Duramax families in particular. On those there is no ignition system to rule out, so the question is fuel or compression only, and the factory balance-rate display is the fastest way to see how large a correction the module is applying to keep cylinder 8 in line.
Common causes
- Marginal fuel supply showing up first at the end of the rail — tiring pump, restricted filter or blocked strainer
- Voltage drop along the injector harness branch delaying opening at the last injector on the run
- Restricted, worn or slow-opening cylinder 8 injector
- Low compression on cylinder 8 from worn rings, a burnt valve or a valvetrain fault
- Worn spark plug or a coil breaking down under load on petrol V8s
- Head gasket leaking between cylinder 8 and its neighbour or into the coolant jacket
- Degraded crankshaft position signal or damaged reluctor wheel causing attribution to land on the wrong cylinder
- Intake plenum or runner gasket leak at the rear of that bank
- Corroded or high-resistance connector at the cylinder 8 injector
Symptoms
- Check engine light with a rough idle that settles as revs rise
- Shake or vibration felt at low RPM and under light load
- Reduced power when towing or climbing, where the fuel supply is most stretched
- Companion misfire codes such as P0308 or P0300
- Fuel economy down noticeably
- Black smoke or hard starting on diesel V8 applications
- Contribution figures that look marginal on cylinder 8's firing-order neighbours as well
- Fault that worsens as the fuel tank level drops, pointing at supply rather than the cylinder
Diagnostic steps
- 1.Read every stored code. An injector circuit code on the same cylinder means the fault is electrical and repairing it usually clears the balance code with it.
- 2.Measure fuel pressure under load before anything else. Cylinder 8 sits at the far end of the rail, so a marginal supply shows here first and looks exactly like a bad cylinder.
- 3.Measure supply voltage at the cylinder 8 injector connector rather than at the rail feed. It is the last injector on the harness branch, so accumulated voltage drop lands here.
- 4.Look up the firing order and check the contribution figures for the cylinders that fire immediately before and after cylinder 8. A smeared deviation across neighbours points at the crank signal, not the cylinder.
- 5.Confirm which physical cylinder this engine calls number 8 from the cylinder diagram. GM V8s put it at the rear of the passenger bank, Ford modular V8s at the rear of the driver's bank.
- 6.Run the scan tool's balance or contribution test and record the size of the deviation. A large repeatable figure is meaningful; a small wandering one usually is not.
- 7.On petrol engines, pull the cylinder 8 plug and compare it against the others, then swap the coil with a neighbour and see whether the fault moves.
- 8.Compression- and leak-down-test cylinder 8 before buying any fuel component. On the rearmost injector of a V8 a wrong guess costs the teardown as well as the part.
- 9.Smoke-test the intake at the rear of that bank for a plenum or runner gasket leak.
- 10.If supply, ignition and compression are all proven, swap the cylinder 8 injector with one from a healthy cylinder and see whether the fault follows it.
Repair cost
$100 – $3,500
Diagnosis is $100 to $200 and is worth doing properly here, because the two cheap checks — rail pressure under load and voltage at the injector connector — occasionally end the job before any part is bought. A plug or coil is $100 to $350 fitted. A petrol injector is $60 to $400 in parts, but cylinder 8 sits at the top of the labour range on nearly every V8 because it is under the upper intake plenum and the gaskets come with it. Diesel injectors run $400 to $900 each fitted and a full set on a high-mileage Power Stroke or Duramax reaches $2,000 to $3,500. Supply-side repairs are $150 to $800 and a high-pressure pump is $600 to $1,600. Mechanical causes start near $1,200 and can exceed $3,000.
Estimate your repair
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DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.