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2002 Ford Explorer 4x4 4.0L OBD2 Code P0133 - O2 Sensor #1 Issue After Replacing All Sensors

Model: 2002 ford explorer 4x4 4.0L Fault Code: P0133 Posted: 2017-07-20 12:00

I'm experiencing a persistent 'Service Engine Soon' warning light on my 2002 Ford Explorer 4x4 with a 4.0L engine, and the fault code displayed is P0133 — indicating an issue with O2 sensor #1. I have already replaced all four oxygen sensors, upgraded the power pack from Pugs to E3-style connectors, inspected and replaced all wiring harnesses, changed both the oil and oil filter. I've also replaced the fuel cap. Despite these efforts, the 'Service Engine Soon' light continues to come on. I took my vehicle to a private mechanic who performed all the work — I purchased all parts myself. I'm now wondering if it's possible that one of the oxygen sensors was defective from the start or if there's another underlying issue causing this recurring fault code. Could the O2 sensor be faulty even after replacement? Or is there something else I should check?

Related fault codes
P0133
Comments (2)
Anonymous 2017-07-20 12:45

Are there any additional diagnostic trouble codes (DTCs) present? Could it be a manufacturer-specific P11xx code? Is the vehicle stock configuration with no modifications? The O2 sensor monitor detects abnormal response — you should check actual sensor voltage readings. Please provide more details about your sensor signals and other parameters like ECT, manifold pressure, MAF/MAP for further diagnosis. Also, ensure that the E3 connector is correct — Ford recommends Motorcraft-style connectors, not generic E3 plugs. Confirming this with the owner's manual or dealership documentation is advised. Any additional codes? Vehicle stock? PS: Is your air filter a K&N type?

Anonymous 2017-07-20 13:03

Let's clarify — you need to monitor actual O2 sensor voltage readings. Are the voltages stable or fluctuating? Try intentionally introducing lean or rich conditions (e.g., by adjusting fuel mixture) and observe how the sensor responds. With data such as O2 sensor voltage, engine coolant temperature (ECT), manifold pressure, MAF/MAP inputs, we can identify potential issues like fuel delivery, ignition coil performance, or ECU calibration. Again: confirm that you're using the correct Ford Motorcraft-style oxygen sensor connectors — generic 'E3' plugs may not be compatible with this model year. Always refer to the official Ford service manual or dealer documentation for proper parts. Are there any other codes present? Is the vehicle stock? PS: Has your air filter been replaced with a K&N filter?