Guided mode

System too lean, bank 1 (P0171)

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0 Completed steps

5 Total steps

0% Progress

Step 1 of 5

Read the fuel trims in three conditions — Current step
Check airflow measurement and fuel supply — Not started
Hunt for unmetered air — Not started
Check the exhaust and the oxygen sensor — Not started
Confirm the repair — Not started

Diagnostic

Read the fuel trims in three conditions

Step 1 of 5

Record long- and short-term fuel trims at idle, at 2500 rpm with no load, and under load with the vehicle driven or on a dynamometer. Compare bank 1 with bank 2 where the engine has two banks.

A clear pattern: lean at idle only, lean across the range, or lean under load only.

What did you find?

Pick the outcome you actually observed. The flow follows the branch the author wrote for it.

Lean at idle and correct at speed Caution
Classic pattern for unmetered air: a vacuum or intake leak matters most when airflow is low.
Lean across the whole range Caution
Suspect measured airflow or fuel supply rather than a small leak.
Lean under load only Fault
Fuel delivery under demand: pump, filter or pressure regulation.

Your notes

Measurement (%)

Completed steps

Read the fuel trims in three conditions Current step
Check airflow measurement and fuel supply Not started
Hunt for unmetered air Not started
Check the exhaust and the oxygen sensor Not started
Confirm the repair Not started

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