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 coolant outlet flange (a molded plastic housing at the rear/transmission end of the cylinder head) and the two-pin coolant temperature sensor mounted in it are a classic, high-frequency VW failure. The plastic flange warps and cracks under constant heat and pressure, and the sensor's plastic body and O-ring degrade, both causing a slow coolant leak from the back of the engine, often hard to see. On the VR6 the flange is buried and awkward to reach. Loss of coolant leads to overheating; a failing blue temp sensor also causes poor cold-start running and erratic gauge/fan behavior.

MK3 Golf/GTI models use a plastic coolant flange/housing on the cylinder head that warps and cracks from repeated heat cycling, and the 'blue' coolant temperature sensor that lives in it also fails intermittently. The flange leaks coolant, while a failing sensor feeds bad data to the ECU (rich/lean running, fluctuating gauge, hard cold starts, coolant warning light). Affects both the 2.0 8v and VR6. Note the black sensor drives the cluster gauge and the blue sensor feeds the ECU.

The early black-topped coolant temperature sensor (G62, often on the coolant flange below the upper radiator hose) is a well-known weak point on the 2.0 8V and 1.8T New Beetle. The plastic sensor and its flange crack or the sensor sends erratic readings, causing the temperature gauge to swing or read falsely hot/cold, hard cold starts and rich/lean running, and a check-engine light. The original black sensor was superseded by an updated green sensor. Because the ECU uses this signal for fueling and fan control, a failed sensor can mimic a thermostat or head-gasket problem and trigger unnecessary repairs.

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.