OBD-II trouble code
P219A: Bank 1 Air/Fuel Ratio Imbalance
One cylinder on bank 1 is burning a noticeably different mixture from its neighbours. This is not a misfire code and it does not name the cylinder — the monitor can tell that the bank is uneven but it cannot tell you which cylinder or why, and understanding that limit is what keeps the diagnosis cheap.
Quick facts
- System
- Powertrain
- Category
- Fuel & Air
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $1,800
- DIY difficulty
- Advanced DIY
Browse every code in P2100–P219A, or start from the full code library.
What does P219A mean?
P219A says the cylinders on bank 1 are not all burning the same air-fuel mixture. On a V engine, bank 1 is the bank containing cylinder number one; on an inline engine it is the whole engine. The important thing to grasp is how the computer works this out, because it explains exactly what the code can and cannot tell you.
There is normally one wide-range oxygen sensor ahead of the catalyst on each bank, and every cylinder's exhaust passes it in firing order. A healthy engine produces a repeatable ripple in that sensor's signal as the pulses go by. GM's published description of this diagnostic is that it watches the pre-catalyst sensor's signal frequency and amplitude by accumulating voltage over a fixed sample period, and flags an imbalance when several consecutive samples of that accumulated voltage sit consistently above the expected value. In plain terms: one cylinder that is off distorts the signal once per engine cycle, and the monitor is looking for that recurring distortion rather than for an absolute mixture error.
This is why the code behaves the way it does. A single shared sensor can detect that the ripple is wrong, but it cannot separate one cylinder from another, because every pulse arrives at the same sensor. Attributing the fault to a specific cylinder requires a different technique entirely — deliberately perturbing the fuelling of one cylinder at a time and timing the sensor's response, which is what the cylinder-specific codes in this family use. So P219A tells you the bank is uneven. It does not tell you which cylinder, whether that cylinder is rich or lean, or what the cause is. A leaking injector, a vacuum leak drawn by one runner, a weak spark on one cylinder, and a cylinder with low compression all look identical from here.
The monitor is also conditional. GM's run criteria require closed-loop operation, system voltage between 10 and 32 volts for more than four seconds, coolant above minus 20 Celsius, engine speed between 950 and 2750 rpm, and mass airflow between 1 and 600 grams per second, with no related codes already set. Those numbers are the GM example rather than a universal standard, but the shape generalises: this test does not run at idle alone and it does not run on a cold engine. A repair verified only by idling in the bay has not been verified at all.
Common causes
- Vacuum leak drawn into one or two cylinders — intake manifold gasket, runner gasket, or hardened injector O-ring
- Crankcase ventilation fault, a split PCV hose, or a stuck PCV valve
- Carbon or varnish deposits on one direct-injection injector restricting its spray
- Exhaust leak forward of the catalyst letting outside air reach the sensor
- Worn spark plug or weak coil on one cylinder
- Leaking or sticking fuel injector
- Intake valve deposits on a direct-injection engine affecting one port
- Fuel contamination, or fuel diluting the engine oil
- Slow or lazy wide-range air-fuel sensor distorting the signal the monitor reads
- Uneven compression from a sticking valve or lifter, which is real but uncommon
Symptoms
- Check engine light, often with no obvious driveability complaint at first
- Slightly rough or uneven idle
- Hesitation or a stumble under light load
- Worse fuel economy than usual
- Fuel trim values that look odd on one bank
- Sometimes lean, rich, or misfire codes stored alongside it
- In severe cases a hard start, though the engine normally runs
Diagnostic steps
- 1.Record the freeze frame first, specifically the engine speed and load at which it set, because you will need to reproduce those conditions to verify the repair. Fix any other stored codes before chasing this one — the monitor will not even run with certain related codes present.
- 2.Check intake manifold pressure at a warm idle in park or neutral against the specification for the engine; one published GM figure is 20 to 48 kPa. A reading outside it points at an induction or mechanical problem to settle before going further.
- 3.Inspect the air induction system for leaks and restrictions with the engine off — the airbox, the duct to the throttle body, and every joint between the mass airflow sensor and the intake.
- 4.Check the crankcase ventilation system. A split PCV hose or stuck valve is cheap, common, and routinely overlooked on this code.
- 5.Smoke-test the intake for vacuum leaks with the engine running, paying attention to the injector O-rings and individual runner gaskets, because a leak at one runner affects one cylinder and that is precisely what this code detects. Do not spray flammable carburettor cleaner near hot exhaust or the ignition system to hunt leaks; use smoke or an unignitable method.
- 6.Check for exhaust leaks forward of the catalytic converter with the engine running. A pinhole ahead of the sensor pulls in outside air and distorts the signal the monitor is reading. Keep clear of the exhaust, which is hot, and of the belts and fan.
- 7.Compare ignition components across the bank — plug condition and coil output. Measure coil or injector resistance only with the component disconnected and the ignition off, and keep away from a running ignition system, which carries enough voltage to injure.
- 8.Perform an injector balance or contribution test per the manufacturer procedure, and check the oil for fuel dilution. On a direct-injection engine the fuel rail runs at thousands of psi, so relieve the pressure per the factory procedure before opening any line and keep ignition sources away.
- 9.Do not try to test the wide-range air-fuel sensor with an ohmmeter. These are heated, actively controlled sensors and a resistance reading across them says nothing about whether the signal is accurate; evaluate them with scan-tool live data or an oscilloscope instead.
- 10.Finish with a compression or cylinder leak-down test if everything upstream is good — engine off, plugs out, fuel system disabled per the factory procedure. This is last because it is the most labour and the least likely answer.
- 11.Verify the repair by driving the vehicle back through the speed and load recorded in the freeze frame, in closed loop and within the monitor's rpm and airflow window. A short idle test cannot satisfy those conditions and will leave the monitor unrun rather than passed.
Repair cost
$150 – $1,800
Cost is decided entirely by which of the causes it turns out to be, and the cheap ones are well represented. A PCV hose, a gasket, or a spark plug set is $150-$400 with diagnostic time. An intake manifold or runner gasket runs $250-$700 depending on access. Induction cleaning for injector or valve deposits is typically $200-$500. Replacing a direct-injection injector is the expensive common outcome at roughly $400-$900 for one, and a full set on some engines runs toward the top of this range; shops sometimes propose a complete set when one is at fault, which is worth questioning. Mechanical repair for a sticking valve or low compression exceeds this range. Published cost data specific to this code is thin, so these are planning figures. Left alone, a persistent imbalance can damage the catalytic converter, which is a far larger bill than any of the above.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with fuel injector replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.