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P0118 Fault Code: High Water Temperature Sensor Issue on Cummins 5.9 (2003) with 4WD and Auto Transmission

Model: Cummins 5.9 Fault Code: P0118 Posted: 2018-09-30 13:21

I made a critical error while driving up the Rockies towing a small vehicle. My engine temperature spiked dramatically, and the cooling fan failed to activate. Assuming the two-wire coolant temperature sensor controlled the fan operation, I incorrectly jumped the wires together without installing a fuse—using a paperclip to short the 4.5V signal from the sensor to ground. After this action, the temperature gauge reads 'max' hot even when the engine is cold, indicating no proper gauge function. The gauge fluctuates between max and lower readings randomly. Occasionally, the check gauges warning light activates, and the fan appears to operate intermittently—though I can hear it running at high speed. I performed a dashboard diagnostic check, which showed all systems as normal. However, the P0118 fault code persists. Did this action damage the ECU? Is the system repairable or does it require replacement of the sensor or wiring? Additional symptoms include erratic idle, fluctuating oil pressure, and a noticeable throttle sensation when the check gauges light cycles on and off during driving—especially at highway speeds with cruise control engaged.

Related fault codes
P0118
Comments (3)
Anonymous 2018-10-04 12:35

Remove the wire jumper immediately. Clear the P0118 code using an OBD2 scanner, then drive normally to see if the fault returns. Do not rely on dashboard checks—use a proper diagnostic tool to read live sensor data and verify actual engine temperature signals.

Anonymous 2018-10-05 19:46

I use an Edge tuner; I cleared the P0118 code, but it returned immediately. I removed the jumper as soon as I noticed the gauge reading 'max' hot. Since then, I've experienced erratic idle, persistent check gauges light, fluctuating oil pressure, and the water temperature gauge cycling between high and low readings while driving. I can feel a noticeable change in throttle response when the check gauges light flashes—this occurs regularly during highway cruising. A small orange engine icon remains lit after startup.

Anonymous 2018-10-05 19:49

While cruising at 75 mph with cruise control engaged, I notice the check gauges light cycles on and off, and this coincides exactly with fluctuations in idle speed—matching the pattern of my engine temperature gauge. This suggests a sensor or ECU communication issue tied to the coolant temperature signal.