Diesel Trouble Codes

P0118 — Engine Coolant Temperature Circuit High Input (7.3 Power Stroke)

The PCM sees high voltage/low resistance on the ECT circuit, interpreting it as an extremely low coolant temperature. On the 7.3L Power Stroke the ECT is used…

Updated September 2, 2026

What it means

The PCM detects an abnormally high voltage or resistance in the engine coolant temperature sensor circuit, indicating the sensor signal is out of expected range. Typical on this platform: 7.3L Power Stroke 1999-2003 Super Duty/Excursion; OBS 1994.5-1997. HEUI, no common rail, no VGT, no EGR/DPF/SCR, gear-driven timing, CMP only for speed/position, EBP for warm-up, IDM/UVCH known issues.1

Most likely causes

  1. ECT sensor internal open or failed thermistor — Open in the sensor raises circuit voltage; PCM reads very low temperature. Diagnose by checking ECT PID vs. a known-good thermometer at the thermostat housing and inspecting sensor connector for corrosion.1
  2. ECT signal wire open or high resistance to PCM — Chafed/broken signal wire or poor pin contact raises voltage. Inspect harness from sensor to PCM, wiggle-test while watching ECT PID.1
  3. ECT reference voltage or ground fault affecting the sensor circuit — Shared 5 V reference or sensor ground issues can bias the ECT reading high. Verify 5 V reference and sensor ground integrity at the PCM connector and compare to other temperature sensors.1
  4. Connector corrosion/moisture at the ECT sensor — Coolant seepage or external moisture increases resistance at the sensor terminals. Inspect connector for green/white corrosion and coolant contamination.1
  5. PCM input fault for ECT — Rare internal PCM fault causing high input reading. Confirm wiring and sensor are good before suspecting PCM.1

How to diagnose

  1. Verify freeze frame and confirm P0118 is present with ECT PID reading implausibly low (e.g., -40°F).
  2. Key on, engine off: check ECT sensor resistance to spec at known temperature; compare to a thermometer.
  3. Inspect ECT connector and wiring for corrosion, coolant intrusion, chafing, and proper pin engagement.
  4. Measure reference voltage and sensor ground at the ECT connector; compare to other sensors.
  5. Perform a wiggle test on the ECT harness while monitoring live ECT PID for dropouts.
  6. If wiring and sensor test good, check PCM connector pins for corrosion and terminal tension.
  7. Clear codes, road test cold to hot, confirm ECT rises smoothly with engine temperature.

Parts and tools you may need

Rough cost and difficulty

ECT sensor is inexpensive; labor is minimal if accessible. Wiring repairs vary with harness damage extent. Diagnostic time dominates cost.

When to see a pro

If ECT PID stays at -40°F after sensor and harness checks, or if multiple temperature-related codes appear with erratic 5 V reference, have a diesel technician evaluate PCM power/ground and internal inputs. Do not attempt high-pressure oil or fuel system repairs for this code.

Sources

  1. Curated DTC seed — 7.3 Power Stroke (curated) ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎