Persistent P0171 DTC

  • 3.4L
  • 4WD
  • 240,000 MILES
The engine is factory supercharged w/ K&N breather filter. Cannot pass smog.

P0171 DTC, (bank 1 lean condition), STFT1 +/- 10%, LTFT1 39, 8%, LTFT2 98%. (False lean condition?), LTFT1 will drop to +/- 10% only when accelerating at freeway speed. (Ex; passing speed). No change at idle or wide open when parked.

Engine vacuum is rock steady 21 lbs, no pirate air detected, vacuum and breather hoses intact and A-OK. MAF sensor, upstream F/A ratio sensor and downstream O2 sensor replaced with OE parts. P0171 persists.

Detected malfunctioning thermostat - replaced. (Red herring?)

The new MAF caused an increase in gas mileage of 50+% but returned to normal after 7 days.

The engine purrs like a 3rd generation fz-5e should, oil is not consumed or contaminated. Catalytic converter inlet and outlet temps within 100 degrees F of each other. The outlet is warmer.

I tried re-setting the DTC with AL519 diagnostic tool and by disconnecting the negative terminal during repairs - P0171 persists.

I had the same issue 1 yr ago. Replaced plugs, wires, and ignitors. This resolved the P0171 DTC until now.

I just discovered (by reading owners manual) I installed the wrong type spark plug last year. OE is a 2 ground electrode (ngk 3289) - says so in big block letters.

Could the plugs create a (false) lean condition?
What am I missing?

I've ordered the correct type of plugs for replacement.

I sure could use some professional guidance.
Do you
have the same problem?
Thursday, September 19th, 2019 AT 3:20 PM

4 Replies

Welcome to 2CarPros.

This is a tough one here. Since it is reading lean, are you certain there is no slight exhaust leak? A leak at the manifold or before the converter will cause a lean condition on one bank.

Here is the diagnostic flow chart for the code. The attached pictures correlate with the directions.


2001 Toyota Truck 4 Runner SR5 4WD V6-3.4L (5VZ-FE)
Vehicle ALL Diagnostic Trouble Codes ( DTC ) Testing and Inspection P Code Charts P0171 Troubleshooting

DTC Detecting Condition

pic 1

Steps 1 - 4

pic 2

Fuel trim refers to the feedback compensation value compared to the basic injection time. Fuel trim includes short-term fuel trim and long-term fuel trim.

Short-term fuel trim is the short-term fuel compensation used to maintain the air-fuel ratio at its ideal theoretical value. The signal from the A/F sensor is approximately proportional to the existing air-fuel ratio, and ECM comparing it with the ideal theoretical value, the ECM reduces fuel volume immediately if the air-fuel ratio is rich and increases fuel volume if it is lean.

Long-term fuel trim compensates for the deviation from the central value of the short-term fuel trim stored up by each engine tolerance, and the deviation from the central value due to the passage of time and changes of environment.

If both the short-term fuel trim and long-term fuel trim exceed a certain value, it is detected as a malfunction and the MIL lights up.

- When the DTC P0171 is recorded, the actual air-fuel ratio is on the LEAN side. When DTC P0172 is recorded, the actual air-fuel ratio is on the RICH side.
- If the vehicle runs out of fuel, the air-fuel ratio is lean and DTC P0171 is recorded. The MIL then comes on.
- If the total of the short-term fuel trim value and long-term fuel trim value is within 35% (80 C (176 F) or more), the system is functioning normally.
- The A/F sensor (bank 1 sensor 1) output voltage and the short-term fuel trim value can be read using the OBD II scan tool or TOYOTA hand-held tester.
- The ECM controls the voltage of the AF1 - and AF1 - terminals of the ECM to the fixed voltage. Therefore, it is impossible to confirm the A/F sensor output voltage without OBD II scan tool or TOYOTA hand-held tester.
- OBD II scan tool (excluding TOYOTA hand-held tester) displays the one fifth of the A/F sensor output voltage which is displayed on the TOYOTA hand-held tester.

Wiring Diagram

pic 3


pic 4

1. Connect the TOYOTA hand-held tester to the DLC3.
2. Switch the TOYOTA hand-held tester from the normal mode to the check mode.
3. Start the engine and warm it up with all the accessory switches OFF.
4. Drive the vehicle at 60 - 120 km/h (38 - 75 mph) and the engine speed at 1,600 - 3,200 rpm for 3 - 5 min.

HINT: If a malfunction exists, the MIL will light up during step (4).

- If the conditions in this test are not strictly followed, detection of the malfunction will not be possible.
- If you do not have a TOYOTA hand-held tester, turn the ignition switch OFF after performing steps (3) and (4), then perform steps (3) and (4) again.

Steps 5 - 6

pic 5

Step 7

pic 6

Steps 8 - 11

pic 7

Read freeze frame data using TOYOTA hand-held tester or OBD II scan tool. Because freeze frame records the engine conditions when the malfunction is detected. When troubleshooting, it is useful for determining whether the vehicle was running or stopped, the engine was warmed up or not, the air-fuel ratio was lean or rich, etc. At the time of the malfunction.


HINT: TOYOTA hand-held tester only:

By putting the vehicle's ECM in the check mode, 1 trip detection logic is possible instead of 2 trip detection logic and sensitivity to detect open circuits is increased. This makes it easier to detect intermittent problems.

A. Clear the DTCs (See step 2).
B. Set the check mode (See step 3).
C. Perform a simulation test.
D. Check the connector and terminal.
E. Handle the connector


Let me know if that helps.

Was this
Friday, September 20th, 2019 AT 6:38 PM
4runner superchargers were a dealer add on. Spark plugs should not make a difference with fuel trims. Even though your vacuum reading is good, there could still be a vacuum leak. The easiest way that I find them is to monitor the short fuel trims while spraying along the intake and watch for the change in the %. Another possibility is if the calibration is not correct for the supercharger or the programming reverted back to stock, or fuel pressure is insufficient during a load.
Was this
Saturday, September 21st, 2019 AT 1:15 PM
I had dismissed fuel system issues as the code is only bank 1. But now I see the error of my ways. I've done the carb spray trick and seen no changes. Certainly willing to give it another go. Going to test the fuel pressure/vacuum, fuel pressure regulator and electrical continuity.
I'll post my results when complete.
Can the ECM be re-programmed?
Thank you guys, I appreciate the insights.
Was this
Saturday, September 21st, 2019 AT 8:48 PM
The carb spray trick will work on huge vacuum leaks but most of the vacuum leaks that I repair will not make a difference in rpm when you spray the intake, but will certainly show up on the fuel trim pids. As far as programming is concerned, the manufacturer of the supercharger, most likely trd will have insight on the programming.
Was this
Saturday, September 21st, 2019 AT 8:56 PM

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