P1124–P1126 Long Term Fuel Trim, Additive Air, Bank 1 - System too Lean
The engine is running with too much air for the fuel it is getting. An unmetered air leak downstream of the airflow sensor is the usual cause.
Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.
Which code is which
Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.
| Code | Definition | Severity |
|---|---|---|
P1124 | Long Term Fuel Trim, Additive Air, Bank 1 - System too Lean | Investigate soon |
P1126 | Long Term Fuel Trim, Additive Air, Bank 2 - System too Lean | Investigate soon |
The definition
P1124: Long Term Fuel Trim, Additive Air, Bank 1 - System too Lean.
P1124 is a manufacturer-specific powertrain code (the engine or gearbox). Its meaning is set by the carmaker rather than by the SAE standard, so a generic OBD-II reader will often show it as an unknown code or misread it.
What to check first
Cheapest and fastest checks first:
- Find the air leak first. Check the intake boots, the PCV system, the dipstick seal and the oil filler cap before replacing any sensor.
- Smoke-test the intake if you can - it finds in minutes what guessing takes weeks to.
- Only then look at fuel delivery and the airflow sensor.
Reading the code is the easy part
A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.
VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.
$249 once. No subscription, unlimited cars. Windows 10/11.
See VAGPULSE — $249 Look up another codeSoftware downloads instantly · cable ships September 30 · cable guarantee — refund if it won't connect
Prefer to work on your phone? The VAGPULSE Android app is a free download and its demo mode runs against a sample car with no hardware at all, so you can see exactly what it reads before spending anything. Reading your car with it needs the VAGPULSE cable, which comes with the licence.
Common questions
What does P1124 mean?
P1124 means Long Term Fuel Trim, Additive Air, Bank 1 - System too Lean. The engine is running with too much air for the fuel it is getting. An unmetered air leak downstream of the airflow sensor is the usual cause.
Can I drive with P1124?
The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.