2001 Ford Explorer Sport Trac P0174, P0135, and P1131 O2 Sensor Locations and Diagnosis Guide

P0174 refers to the upstream O2 sensor on bank 2. P0135 and P1131 both relate to the same upstream O2 sensor in bank 1. This is a common setup in V6 engines like the 2001 Ford Explorer Sport Trac. For reference, see https://www.obd-codes.com/trouble_codes/. Always verify sensor functionality before replacing — especially heater circuit and signal integrity. The sensor is part of the system detecting lean conditions. Avoid aftermarket sensors with cheap wiring; use OEM or high-quality replacements. Note: Bank 1 is on the passenger side of the engine.

The two upstream O2 sensors are located one on each exhaust manifold (bank 1 and bank 2). The third sensor, downstream, is positioned after the catalytic converter. I plan to install pre-plugged O2 sensors for easier installation.

I'm planning to replace both faulty sensors with Bosch OEM parts — known for reliability and compatibility in Ford vehicles.

I now understand that the two failing O2 sensors are indeed located on the exhaust manifolds, both upstream. This confirms they are upstream sensors (bank 1 and bank 2).

Why do you believe these O2 sensors are faulty? Have you tested the heater circuit for proper operation? Has your vehicle shown a lean condition, such as poor fuel mixture or rough idle?

I replaced both upstream O2 sensors, but after about 20 minutes of driving, the symptoms return — severe engine misfires and loss of power. The issue seems to reappear under load.

Both P0135 and P1131 point to O2 sensor issues. How can I verify if these are actually sensor-related or if another system is at fault?

Could a failing fuel pump be the root cause? When symptoms appear, the truck struggles to take in fuel and loses power significantly.

Diagnostic codes do not confirm sensor failure — they only indicate system issues. The code does not explicitly say 'replace part'. You're making assumptions about component replacement. If you suspect a fuel pump issue, test fuel pressure using a gauge. First, verify the heater circuit and O2 sensor signal function. Then check O2 sensor voltage readings with an OBD2 scanner — monitor long-term and short-term fuel trims (FT), MAP pressure, and other parameters. Also, note that I use standard air filters (not K&N).

No, I use standard factory air filters.

After troubleshooting, the issue was actually a failing fuel pump. The original unit only delivered 40 PSI — far below the required pressure for proper engine operation. This explains the lean condition and misfires under load.