Fuel trim and lean or rich codes
Fuel trim is the percentage the engine computer adds to or takes away from its base fuel amount to keep the air-fuel mixture on target. A positive number means it is adding fuel because the engine was running lean. A negative number means it is taking fuel away.
| Condition | Short-term | Long-term | Total |
|---|---|---|---|
| Warm idle, bank 1 | +4% | +18% | +22% |
| Steady 2,500 rpm, bank 1 | +1% | +5% | +6% |
| Warm idle, bank 2 | +3% | +17% | +20% |
Short-term and long-term trim
Short-term fuel trim changes from moment to moment as the oxygen or air-fuel sensor reports. Long-term fuel trim is the correction the computer has learned over time. A scan tool shows both as percentages for each bank. Add them together for the total correction.
When a correction stays too large for too long, the computer sets a code. P0171 and P0174 mean too lean on bank 1 and bank 2. P0172 and P0175 mean too rich.
Read the pattern
Compare the trims at warm idle with the trims at a steady 2,500 rpm or highway cruise. A big positive correction at idle that shrinks at higher speed often points to air getting in after the airflow sensor, because a small leak matters more when little air is flowing. A correction that stays high everywhere calls for a look at fuel pressure, injectors and airflow measurement.
Then compare the banks. Both banks lean suggests something they share. One bank lean suggests something on that side.
Where unmetered air comes from
GM bulletin 21-NA-105 for 2020–2021 Colorado and Canyon trucks with the 3.6L engine gives a useful checklist before its software update. It lists loose PCV hoses and oil separator connections, a loose oil filler cap, a cracked oil filler adapter and a dipstick that isn’t fully seated. Its companion bulletin, 22-NA-171, traces the same lean codes to a cracked intake boot on 2017–2022 trucks.
Those bulletins apply to those trucks. The places they check are common sources of unmetered air on many engines.
Airflow and downstream codes
An airflow sensor that misreads can push trims off target too. P0101 means the measured airflow didn’t match what the computer expected. The oxygen sensor behind the converter adds a second correction, and P2096 or P2097 means that correction ran out of range.