Question

2002 Ford F150 5.4L with engine code P0128. replaced thermostat and the truck is running worse than it did before

Answer 1

HI there,

There can be other causes for this code. Check the engine coolant temp sensor, the intake air temp sensor, make sure the truck is full of coolant and the fan is working properly. The code basically is due to the engine not reaching normal operating temp fast enough.

Answer 2

P0128 FORD - Coolant Thermostat

Read more: https://www.autocodes.com/p0128_ford.html