98 Ford Navigator 4x4 5.4L Engine Shows P0118 Mode 10 - Sensor Voltage Reads -40°F Despite Expected 5V Output
I'm new to this forum and reaching out for help as I'm planning to retire by year-end—your insights would be greatly appreciated. Recently, I replaced the upper and lower manifold gaskets and the lower manifold due to a vacuum leak. The truck now runs smoothly, but it started displaying an OBD II fault code P0118 in Mode 10 (High Input). The temperature gauge appears to function correctly, suggesting that the issue may lie with the engine coolant temperature (ECT) sensor connected to the Powertrain Control Module (PCM). To troubleshoot, I replaced the ECT sensor. After reconnection and using an OBD II scanner, the same P0118 code returned. The scanner consistently reads -40°F even when the wires are gently wiggled with the engine off or on—no fluctuation. Here's where it gets confusing: When I measured voltage at the sensor connector (with ignition on and connector disconnected), I observed 11.6 volts instead of the expected 5 volts as specified in the factory manual. Does this point to a faulty PCM, or could there be another explanation? I'm seeking advice from experienced users—especially regarding wiring configurations or potential short circuits that might affect the ECT sensor circuit. Thank you for your time and support.
Only this one code? The 5V reference circuit often powers multiple sensors—so if there's a short to ground or 12V, it could cause incorrect readings. This is an ECT (Engine Coolant Temperature) sensor, not a CHT (Coolant Heat Sensor). If possible, sharing the exact location of the sensor and wire colors would be very helpful. I’ll check the wiring diagram later if time permits.