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2002 Dodge Dakota P0138 and P0141 O2 Sensor Fault: Wiring, PCM Issues, and Fix

Model: 2002 Dodge Dakota Fault Code: P0138 Posted: 2011-10-03 18:19

Hello everyone, I own a 2002 Dodge Dakota 4x4 with a 4.7L V8 engine and automatic transmission. This truck is registered in California and equipped with four oxygen sensors and three catalytic converters. I've been dealing with persistent check engine light issues showing codes P0138 and P0141 for several months. Initially, I replaced the O2 sensor with a Bosch unit due to online reports of poor quality—however, the problem persisted. I then switched to an NTK sensor, but that didn't resolve the issue either. After ruling out the oxygen sensors themselves, I tested the wiring harness leading to the sensor. Here are my findings: - Black wire (ground): Reads good — solid ground connection. - Dark Green/Pink wire (heater circuit from relay): Measures 0.049V. This is significantly below expected 12V; heater circuits should typically read close to 12V when powered. - Black/Light Blue wire (sensor voltage supply): Reads only 0.03V — far below the expected level. All other sensors in this circuit (crankshaft position, camshaft position, TPS) function normally. - Tan/White wire (to PCM): Connection is solid and functional. Based on these readings, I suspect a wiring harness fault. However, before investing time in disassembling the harness, I'd like to hear expert opinions. Additional context: My original PCM failed at some point and was replaced with a custom-flashed unit purchased from eBay. While the truck runs fine now, it's running rich and fuel efficiency has dropped significantly—down to around 14 MPG. I believe this could be related to incorrect emissions programming or sensor signal interference due to faulty hardware or wiring.

Related fault codes
P0138P0141
Comments (3)
Anonymous 2011-10-05 11:05

Bump — the voltage measurements were taken using a multimeter. Ground and PCM connections were tested with resistance checks to ground. The heater circuit and sensor supply readings were obtained while the sensor was unplugged and the engine was running. I'm not sure if this affects interpretation, but wanted to clarify for accuracy.

Anonymous 2011-10-06 06:16

Many 4.7L Dodge Dakotas have had issues with the power steering pressure switch leaking oil into the wiring harness. This oil can contaminate oxygen sensors and even seep into PCM connectors, causing false readings or sensor failure. I recommend removing the PCM connectors and inspecting the entire harness for signs of oil contamination — clean any visible residue thoroughly.

Anonymous 2013-04-05 09:24

I was researching a similar issue on this forum when I realized my own Dakota had never been properly resolved. After trying multiple fixes, I took it to a local Dodge dealership. They first replaced the O2 sensor (Bank 1 Sensor 2), which temporarily fixed the P0138/P0141 codes — but the light returned about a week later with identical codes. Upon scanning the PCM, they discovered that my eBay-fitted PCM was programmed for Federal and Canadian emissions standards, not California emissions. The truck has three catalytic converters and four O2 sensors, which requires specific calibration. The dealership couldn't reflash the unit due to compatibility issues, so I had to purchase a new PCM at $750 plus a $150 reflash fee. It took about two weeks to receive it — in the meantime, I blew a head gasket (which I fixed myself with rebuilt heads). After installing the new PCM, the truck ran normally again. About a month later, my truck died on the way to work and displayed a 'NO BUS' signal. Given that all sensors were checked and ruled out, I had it towed back to Dodge. They replaced the PCM under warranty because they couldn't get it to respond — this third PCM appears to have finally resolved the issue. The dealership experience was expensive but professional — they have full diagnostic tools and expertise for 4.7L Dakota electrical issues. While not ideal due to my anxiety about bringing a vehicle in, overall it wasn’t terrible. Thanks to Jeff for pointing me toward the right troubleshooting path; I personally inspected both the harness and pressure switch, and found no visible oil contamination at that time.