P0171 means the control unit has detected signs of a lean mixture in bank 1. P0174 refers to bank 2. A bank is a group of cylinders for which the control unit monitors the mixture separately. First save the codes and their statuses, then compare the fuel trims across the banks. This will make it clearer which check to start with.
If the Check Engine light flashes while the engine is running, the engine shakes severely, or you experience a sudden loss of power, stop in a safe place and get help. If the light is on steadily, check your vehicle's manual and arrange to have the car checked. Read the codes and review the data only after stopping.
What should you check for codes P0171 and P0174?
1. Save the full list of codes
While parked, connect a compatible Bluetooth LE adapter and open the engine codes screen in LLCAR. Save the exact record of P0171 or P0174, any accompanying codes, and the available statuses. Separately, note the mileage, when the warning appeared, and what was happening with the car. If the scanner shows a freeze frame, save it. It may contain the rpm, load, and temperature at the moment the fault was logged. Save the data before clearing the codes so the technician has the conditions under which the problem appeared. How to read engine codes with your phone.
2. Compare the trims for each bank
Take several readings of the warmed-up engine in a single mode where the ECU adjusts the mixture based on exhaust sensors. On a scanner, this mode is usually called "closed loop." For each available bank, write down the LTFT and STFT from the same moment. LTFT shows the accumulated correction, STFT the current one. Add them together as a guideline for comparison. A positive sum indicates added fuel in that mode, while a sum near zero can occur when the trims cancel each other out. The sum cannot tell you the actual mixture composition. If a parameter is missing, leave the field blank. How to read LTFT and STFT.
3. Identify when the deviation repeats
Compare a steady warmed-up idle with readings from a normal drive under load. When repeating a measurement in either mode, look for similar temperature, rpm, load, and fuel-control status. If no safe readings are available, stick to what you already have and pass the data to the service shop. There is no need to load the engine artificially just to get a code. If the vehicle's manual allows a normal drive, wait until you have stopped to look at the screen.
| Recurring pattern | Which check to start with |
|---|---|
| Positive trim is more noticeable in one bank | Start by checking the intake for that bank, possible exhaust leaks upstream of its air–fuel mixture sensor, or its injectors. If those checks find nothing, check the conditions shared by both banks as well. |
| Trims are clearly positive in both banks | First check the intake sections and fuel delivery shared by both banks. It is also useful to check the shared mass airflow sensor readings under the same conditions. |
| The deviation is stronger at idle | Discuss an intake leak test with the technician. This pattern helps choose a direction, while the leak location is found through a separate check. |
| The deviation grows under load | Compare the fuel delivery and airflow readings following the vehicle's procedure. Two readings in a similar mode are more useful than a random spike. |
4. Confirm the cause by measurement
In LLCAR AI0.19 for Android, open "Control" → "Diagnostic rules". That section explains separate fuel-trim checks, including cases where STFT and LTFT have opposite signs and comparisons between idle and driving under load. They help you read the data pattern and choose a check. The code and the trims come from one control unit, so a component check is best done separately. For example, a technician can smoke-test the intake for leaks, measure fuel pressure, or check the mass airflow sensor signal and power supply against the vehicle documentation. As a starting point, you can mention whether the air hose, filter housing, or connector was touched before the code appeared. How to check mass airflow readings (in Russian) · What LLCAR shows (in Russian).
5. After the check, read the codes again
Compare the new readings with the original ones in the same mode. Note which code statuses the scanner returned and under what conditions. If the code reappeared, save that result together with the trims. If it disappeared right after clearing, let the vehicle's diagnostics check the system again under the conditions specified by the manufacturer. A light that has gone out, an empty scanner response, and a complete scan that finds no code all mean different things. How to tell code statuses apart after a check.
What one Highlander reading showed
P0171 and P0174 appear in the Toyota Highlander records as early as 27 October. On 6 November the contaminated mass airflow sensor was replaced with a new one, and on 7 November the oil and air filter were changed. In the records from 9–10 November both codes were still present. These saved lists do not establish when the ECU set either code again.
In one reading taken while parked on 7 November, LTFT for the first bank was +15,625% and STFT was −15,625%. The commas are decimal separators, and the sum was 0 percentage points. The driver could have seen a large long-term trim and missed the other one. This reading shows how the trims canceled each other out while parked. To assess operation under load, a separate set of readings is needed. Highlander measurements after maintenance.
Quick answers
What if I see only P0171? Start with bank 1. On an engine with two banks, compare the trims of both in the same mode. Check the engine documentation to confirm which cylinders and components belong to the first bank.
If the light went out, why read the codes again? The scanner may still show a stored, pending, or permanent code if the ECU supports these modes. Write down its status and the time of the reading. This helps tell history apart from a recurring sign.
Why not replace the mass airflow sensor right away? An air leak or fuel delivery problems can produce similar symptoms. Before replacing it, ask for the sensor signal to be checked and the suspected circuit tested separately.
Save one set of readings for each operating condition to share with the technician. Start with the code and status, LTFT and STFT per bank, rpm, load, and the conditions under which the symptom appeared.