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P0155 Fault Code in 1997 Ford Ranger 3.0L: Diagnosing HO2 Sensor Heater Circuit Issue

Model: 1997 ford ranger Fault Code: P0155 Posted: 2015-10-06 09:34

Hello, I'm troubleshooting a P0155 fault code on my 1997 Ford Ranger with a 3.0L engine — this indicates a heater circuit issue for the oxygen sensor in Bank Two, Sensor One. I recently replaced the HO2 sensor as per standard procedure. To verify power and ground connections, I checked for 12V power at the key-on state, which was present. I also tested for a proper ground connection from the Power Control Module (PCM), but instead of a ground, I measured 12V — resulting in two separate 12V supplies. I then tested Bank One as well to compare. When both oxygen sensors were disconnected, I observed correct 12V power and a proper ground signal at the expected locations. However, when I reconnected one of the sensors (Bank Two, Sensor One), the ground signal unexpectedly converted to 12V positive. This behavior is not typical — it suggests either a wiring fault or a potential issue with the PCM. Is this normal behavior for a 1997 Ford Ranger? Or does it point to a faulty PCM or a short in the heater circuit wiring?

Related fault codes
P0155
Comments (2)
Anonymous 2015-10-06 09:39

Could this indicate a short circuit between the heater wire and ground in the sensor harness? Are you confirming that both heater wires show 12V when probed with the connector connected to the O2 sensor (Bank Two, Sensor One)?

Anonymous 2015-10-09 17:57

You have 12 volts on both heater wires when probing back with the electrical connector connected to the O2 sensor — is this consistent across both sensors or only in Bank Two?