P0118 Diagnostic Issue: 2004 Dodge Ram 3500 Cummins 4x4 Engine Temperature Gauge Malfunction
I own a 2004 Dodge Ram 3500 Cummins 4x4 with a persistent issue involving the engine temperature gauge. As soon as the key is turned on, the gauge reads full 'hot' regardless of actual engine temperature. Live data from an OBD-II scanner consistently shows 411°F, irrespective of real-time engine conditions. I have already replaced both the coolant temperature sensor and a known-good replacement sensor with no improvement. I've also swapped out the engine Powertrain Control Module (PCM) with two fully operational, running trucks — same result. Additionally, I’ve tested and replaced the instrument cluster panel, which still displays the incorrect reading. Voltage measurements at the sensor terminal show 4.99 volts using a digital multimeter (DVOM), and ground continuity is confirmed with no resistance issues. The wiring harness between the gauge module and PCM shows good continuity, with no shorts to power or ground. I have also replaced the integrated power module (fuse block) without any change in behavior. This issue appears consistent across both my 2004 and a 2003 model Ram 3500 Cummins — suggesting it's not a year-specific fault. Given that all sensor, PCM, and wiring components have been tested and replaced, I'm at a loss as to the root cause. Any insights or troubleshooting steps would be greatly appreciated, especially regarding potential open circuits, faulty ground paths, or scanner tool limitations. Note: The P0118 code is triggered under these conditions — this may indicate an incorrect signal from the sensor or a misinterpretation by the PCM due to improper grounding or signal integrity.
Perform a voltage drop test using wig pins (or T-pins). Connect the negative lead of your digital multimeter directly to the battery negative terminal. Insert one T-pin into terminal A (ground) and another into terminal B (signal), while keeping the thermistor connector attached. Terminal A is typically red/white or brown/white — this should be grounded. Terminal B is usually tan/black or violet/orange — this is the signal wire. With the key on and engine off, measure voltage: - Terminal A should read less than 0.10 volts (100 mV). - Terminal B should show between 0.5–1.5 V when coolant is cold, or 2.5–4.5 V when hot. If terminal B reads exactly 5 volts, this suggests an open circuit in the signal wire, a broken ground connection, a faulty sensor, or a damaged connector. If you observe 5V on the signal wire with the scan tool connected, it could also indicate an open signal path — verify by probing directly at the connector. Please share your voltage readings and specify what type of OBD-II scanner you're using (e.g., standard, advanced, or diagnostic software). This will help determine whether the issue lies in hardware or software interpretation.