Leaking or dripping fuel injector on bank 1, allowing extra fuel into the cylinder; High fuel pressure from a faulty fuel pressure regulator or restricted return line; Contaminated mass air flow sensor reporting less air than actually enters the engine; Evaporative emissions purge valve stuck open, drawing fuel vapour into the intake; Worn spark plugs or ignition misfire causing unburned fuel to reach the oxygen sensor; Leaking intake manifold gasket or vacuum line admitting unmetered air past the sensor; Faulty upstream oxygen sensor biased rich, causing the ECU to overcorrect

DTC codes / Volkswagen codes / P1123

P1123 · VolkswagenLong Term Fuel Trim Add Air Bank 1 System Too Rich

Q: What does code P1123 mean?

Long Term Fuel Trim Add Air Bank 1 System Too Rich. P1123 on Volkswagen vehicles means the engine control module has detected that the long-term fuel trim on bank 1 has reached its rich limit while adding air, indicating the engine is running too rich and the computer cannot lean it out further. The fault is stored when the additive long-term trim value exceeds the calibrated threshold for a set time, typically because unmetered fuel or a false air signal is skewing the mixture.

P1123 on Volkswagen vehicles means the engine control module has detected that the long-term fuel trim on bank 1 has reached its rich limit while adding air, indicating the engine is running too rich and the computer cannot lean it out further. The fault is stored when the additive long-term trim value exceeds the calibrated threshold for a set time, typically because unmetered fuel or a false air signal is skewing the mixture.

SystemPowertrain

Code typeVolkswagen specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Slightly rough idle or idle surge when the engine is cold or under light load
Black sooty exhaust smoke or a strong fuel smell at idle
Reduced fuel economy and frequent refuelling
Hesitation or stumble on acceleration from a stop
Elevated hydrocarbon emissions that may fail an emissions test

Common causes, most likely first

Leaking or dripping fuel injector on bank 1, allowing extra fuel into the cylinder
High fuel pressure from a faulty fuel pressure regulator or restricted return line
Contaminated mass air flow sensor reporting less air than actually enters the engine
Evaporative emissions purge valve stuck open, drawing fuel vapour into the intake
Worn spark plugs or ignition misfire causing unburned fuel to reach the oxygen sensor
Leaking intake manifold gasket or vacuum line admitting unmetered air past the sensor
Faulty upstream oxygen sensor biased rich, causing the ECU to overcorrect

How to diagnose and fix it

Connect a scan tool and record live data: long-term fuel trim, mass air flow, oxygen sensor voltage, and fuel pressure if available
Inspect the air intake tract and vacuum lines for cracks, loose clamps, or disconnected hoses that admit unmetered air
Check fuel pressure with a gauge and compare to Volkswagen specification; inspect the pressure regulator and return line
Test the evaporative emissions purge valve for sticking open by commanding it closed and watching fuel trim
Perform a cylinder balance or injector flow test to identify a leaking or dripping injector on bank 1
Inspect spark plugs and ignition coils for wear or misfire, and repair any ignition fault before retesting
Clear the fault, road test under the conditions that set it, and verify long-term fuel trim returns to a normal range

What a rich bank 1 trim really tells you

A stuck-open evap purge valve is one of the most commonly missed causes because it feeds fuel vapour into the intake without any visible leak or fuel pressure change.
P1123 does not automatically mean the oxygen sensor is bad; a biased sensor can cause the ECU to overcorrect, but replacing it without checking fuel trim first often leaves the code returning.
Volkswagen long-term additive trim is learned at idle and low load, so a vacuum leak that leans the mixture at idle can paradoxically drive the additive trim rich as the ECU compensates elsewhere.
A misfiring cylinder on bank 1 dumps unburned fuel into the exhaust, which the oxygen sensor reads as rich and the ECU tries to lean out, setting P1123 even though the root cause is ignition.
After any repair, the long-term trim must be cleared and relearned through a full drive cycle; simply clearing the code and idling the engine will not confirm the fix.

Can I keep driving with P1123?

You can drive the vehicle short distances to a workshop, but avoid hard acceleration and prolonged idling because the rich mixture can overheat and damage the catalytic converter. Have the fault diagnosed promptly, especially if you notice black smoke or a strong fuel smell.

Tools that help

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