P0118: How to Diagnose & Fix — Engine Coolant Temperature Sensor Circuit High

The 3.5L Duratec and 3.5L EcoBoost V6 engines in the Ford Flex have a critical design flaw: the water pump is driven by the timing chain and mounted internally inside the engine. When the water pump seal fails, coolant leaks directly into the engine oil, creating a milky sludge that destroys engine bearings in a matter of miles. Because the pump is internal, there is no external leak to warn the owner — the first symptom is often catastrophic engine failure. This is one of the most expensive and common failures on high-mileage Flex vehicles, with repair quotes ranging from $5,000 to $7,000+ for engine replacement.

The original black-housing dual coolant temperature sensor (G62) mounted in the coolant flange at the back of the cylinder head develops cracks and fails internally. The temperature sensor G2 (also known as G62) for the gauge has likely failed and is relatively easy to fix; it is located in the coolant flange at the back of the cylinder head and the early square-connector style and later elongated D-shaped black connector sensors are known problematic units. When it fails the ECU sees an implausible coolant temperature, leading to cold starting difficulties at low temperatures, poor idle and acceleration during warm up as well as an erratic or dead temperature gauge.

Separate from the well-known water-pump/thermostat-housing leak, MK7-platform Golf R owners report failures of the engine coolant temperature (ECT) sensor and a notably unreliable dashboard temperature gauge. The gauge tends to sit pinned at ~90C and gives little warning before overheating, so coolant loss or a failing sensor can let the engine run dangerously hot without alerting the driver. A faulty ECT sensor also causes erratic readings, false overheat warnings, poor cold-start fueling/economy, and a check engine light. VW updated early sensor revisions.

The EJ25 2.5L horizontally-opposed engine in 1998-2010 Foresters is notorious for external head gasket failure. Unlike typical blown head gaskets that cause overheating, the Subaru EJ25 gaskets fail externally, leaking coolant and oil down the sides of the engine. The failure is caused by the gasket material (composite) breaking down over time, combined with the horizontal orientation placing uneven stress on the gaskets. This is widely considered the most common and well-documented Subaru defect.

Early MK3 Jettas were fitted with an engine coolant temperature (ECT) sensor with a black or blue top that is prone to internal failure. When it sends bad data, the ECU miscalculates fuel and timing, and the dash temperature gauge reads erratically or the cooling fans fail to switch on/off correctly. Volkswagen released an updated, more-reliable green-top sensor (part 059 919 501A), which became the de facto fix carried across MK3/MK4. It was never a formal recall but is one of the most widely recognized VW fixes of the era. Any MK3 still wearing the original blue/black sensor is a strong preventative-replacement candidate.