Faulty or clogged fuel injector on cylinder 8 delivering an incorrect amount of fuel; Vacuum leak at the intake manifold runner or injector O-ring seal specific to cylinder 8; Worn or fouled spark plug on cylinder 8 causing incomplete combustion; Failed ignition coil or plug wire for cylinder 8 producing a weak spark; Mechanical issue such as low compression, a sticking valve, or a worn camshaft lobe on cylinder 8; Contaminated engine oil or fuel affecting injector performance on that cylinder; Exhaust leak near the cylinder 8 port affecting the oxygen sensor signal used during the balance test

DTC codes / P23xx · Ignition system and misfire / P231B

P231BCylinder 8 Air-Fuel Ratio Imbalance Adjustment At Limit During Balance

P231B indicates that the powertrain control module has reached the limit of its ability to balance the air-fuel ratio on cylinder 8 during a diagnostic balance test, meaning that cylinder's contribution is too far out of line with the rest of the engine. The computer commands individual cylinder fuel adjustments and monitors crankshaft speed variations, and when the correction for cylinder 8 can no longer bring its power output in line, it stores this code.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaIgnition system and misfire

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Rough idle or a noticeable shake that is worse when the engine is cold
Slight misfire or hesitation during light acceleration
Decreased fuel economy over time as the engine runs richer or leaner to compensate
A faint fuel or exhaust odor from the tailpipe in some cases
Reduced power or sluggish throttle response under load

Common causes, most likely first

Faulty or clogged fuel injector on cylinder 8 delivering an incorrect amount of fuel
Vacuum leak at the intake manifold runner or injector O-ring seal specific to cylinder 8
Worn or fouled spark plug on cylinder 8 causing incomplete combustion
Failed ignition coil or plug wire for cylinder 8 producing a weak spark
Mechanical issue such as low compression, a sticking valve, or a worn camshaft lobe on cylinder 8
Contaminated engine oil or fuel affecting injector performance on that cylinder
Exhaust leak near the cylinder 8 port affecting the oxygen sensor signal used during the balance test

How to diagnose and fix it

Connect a scan tool and record freeze-frame data and any stored misfire counters for cylinder 8 before clearing the code
Inspect the spark plug, ignition coil, and plug boot for cylinder 8, replacing any worn or fouled components
Perform a cylinder balance test or compression test on cylinder 8 and compare the results with adjacent cylinders
Check for vacuum leaks at the intake manifold and injector seals using a smoke machine or carburetor cleaner while watching fuel trim data
Test the cylinder 8 fuel injector for resistance, pulse signal, and spray pattern, and clean or replace it if faulty
Verify fuel pressure and inspect the fuel rail and injector harness for corrosion or damaged wiring
After repairs, clear the code and run the manufacturer's balance test or a road test to confirm the correction stays within limits

What the balance test really reveals

A P231B can set even when the engine feels smooth because the balance test detects subtle cylinder-to-cylinder power differences that are not yet severe enough to cause a noticeable misfire.
Technicians often replace the cylinder 8 injector first, but a weak ignition coil or a small vacuum leak at that runner can produce the same balance limit and cost far less to fix.
The code does not always mean the cylinder is mechanically failing; fuel quality, a dirty injector, or a marginal spark plug can push the adjustment to its limit without any internal engine damage.
Exhaust leaks upstream of the oxygen sensor can skew the air-fuel ratio reading during the balance test and cause a false P231B on an otherwise healthy cylinder.
On engines with active fuel management or cylinder deactivation, a stuck or slow-responding lifter on cylinder 8 can set this code before any traditional misfire code appears.

Can I keep driving with P231B?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy acceleration, towing, or long highway trips until the fault is diagnosed. Continued driving with an uncorrected imbalance can damage the catalytic converter and worsen fuel economy.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.