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 2023+ Sequoia with the i-FORCE MAX twin-turbo V6 hybrid powertrain has experienced cooling system issues related to the added complexity of cooling both the twin-turbo engine and hybrid battery system. Owners report coolant loss with no visible external leak (internal leak at turbo coolant lines), electric cooling fan running continuously at high speed even in mild weather, and occasional overheating warnings during towing. The hybrid battery cooling circuit is separate from the engine cooling circuit and has its own failure modes.

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.

On the 2.2L/2.4L Ecotec the thermostat is integrated into a plastic housing on the back side of the engine facing the firewall, and the water pump sits behind the timing cover on the passenger side. Both are recurring leak points. The rear thermostat housing seal hardens and the housing itself corrodes and seeps, and because the leak is tucked against the firewall the coolant evaporates on the hot block instead of dripping — so owners chase a steadily dropping reservoir with no puddle in the driveway, a complaint that appears over and over on the Cobalt forums. The water pump leaks at its gasket toward the passenger side, and because it is timing-chain driven, a leaking pump can wash the front of the engine and contaminate the chain area. A slow leak that runs the car low leads to overheating and, on the Ecotec, a warped or cracked head.

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.