Does the 99 explorer have a vacuum modulator on the tranny and where would I find the vac hoses off the tranny.

System Too Lean (Bank 2) What the P0174 code means. Code P0174 indicates that there is a lean condition detected by the Engine Control Module (ECM). A lean condition will be created if there is too much air, and not enough fuel, in the air fuel ratio (AFR).
https://www.yourmechanic.com/article/p0174-obd-ii-trouble-code-system-too-lean-bank-2-by-blake-griffin

Have you ever changed the oxygen sensors?

bank 1 sensor 1 lean code. Check vacuum lines first. Could be egr leak also. Most vacuum lines run on the back side of the engine on these. Most common fail is the PCV 90 degree elbow. On the passenger side.

At the age your car is, you may have some cracked vacuum hoses. A vacuum leak can cause your car to run lean. Replacing hoses isn't expensive, only tedious.

could still be a vacuum leak.

I’m happy to assist further over the phone at https://www.6ya.com/expert/josh_8f8ad0a8f7e0e6d3

I would check for a vacuum leak somewhere. Like the post for the pcv hoses and elbows to the intake manifold gaskets not sealing. Check with at scan tool the fuel trims to see how the engine is running. If the long term fuel trim is over 10 % then there is a vacuum leak. It is rare for a lean running bank and at the same time a rich running bank. The only thing that could be possible cause for that would be the mass air flow sensor or the hose to the throttle body is not sealing causing unwanted air to get passed the MAF. These engines have a problem when it is cold the manifold gaskets not sealing and then when the engine is warm to seal and the fuel trims be high when cold and then be normal after it warms up

Faulty Heated Oxygen Sensor (HO2S) Bank 1 Sensor 1
- Vacuum leak
- Faulty Heated Oxygen Sensor (HO2S) Bank 1 Sensor 1 harness is open or shorted
- Faulty Heated Oxygen Sensor (HO2S) Bank 1 Sensor 1 circuit poor electrical connection

The Powertrain Control Module (PCM) monitors the Heated Oxygen Sensor (HO2S) activity for 90 seconds after closed loop operation has begun. The PCM determines if the front HO2S is functioning properly by counting the number of HO2S-1 Lean/Rich (L/R) and Rich/Lean (R/L) switches.

If the Powertrain Control Module (PCM) determines that the front HO2S did not switch enough times, the PCM will set the P1133 code.

replace the o2 sensor

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fuel too lean on bank 1 and 2 check for vacume leaks

It appears that your brake booster system has a major vacuum leak which is causing the engine to go lean and stall. Check the hose between the intake manifold and the brake booster. It may be cracked or loose causing outside air to enter the hose when the brake is applied.

these are lean codes. 171 is system too lean bank 1 and 174 is system too lean bank 2. what this relates to is you probably have a vacuum leak. look under for signs of obvious breaks in vacuum hoses. this is your most likely culprit. happy holidays