It says coolant below norm thermostat operating temp i have been watching the temp in the morning it takes a while but gets up to temp how do i check the temp sensor?

code p0128 refers to coolant thermostat-coolant temp below thermostat regulating temperature ====causes mechanical fault
that probably means that the thermostat is stuck open
As to your question "will it cause a knock?" it is possible because the operating temperature of the engine is below what is required for proper combustion and the knock may be the excessive piston clearance until the piston heats up and expands to take up that clearance . I suggest that the thermostat be check out first and replaced if necessary and see if that stops the noise

If you have replaced the coolant it may have trapped air..you fix this by allowing the vehicle to idle with radiator cap off and allow to get to operation temp ever so ofter squeeze the lower radiator pipe this will help move out trapped air when the thermostat opens it will draw the coolant down just add water when it does..if the thermostat doesnt open you may have a bad or stuck closed thermostat. On bad news side if the temp shoots up under acceleration and drops off to norm on deceleration you either have a blown head gasket or cracked head.

Given the age of the car I'd bet the heater matrix is blocked up.
When the car is warmed up look down the back of the engine and trace 2 hoses about 1.5 in diameter heading through the bulkhead. if these are roughly the same temp. ie hot then the matrix is blocked.
Even with correct coolant 5-8 years / 80k miles is the norm for these.

depending on the particular model and facia layout this can be a mornings work for a good diy or a major task for a competent mechanic.

If the vehicle has been run for any time without the correct level of antifreeze / summer coolant then the matrix will block up.

I presume you have checked the thermostat? When the car is warm, feel the upper and lower main rad hoses. The lower should be cooler than the top. Does the heat reflect the actual engine temp? Does the gauge read low all the time? Is the thermostat stuck open and the engine temp is low?

Finally, if you think the coolant is hot enough and the small hoses are as hot, are the heater flap controls working properly?

if the temp gauge in the dash reads below the norm, I would start by replacing the thermostat.

po128 usually means your engine is not reach a high enough operating temp. the most likely cause is a stuck open thermostat, make sure you have good clean coolant in your vehicle, and replace the thermostat and radiator cap is a good cheap place to start if your into doin it yourself.

P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

Symptoms: You will likely not notice any drivability problems other than the MIL illumination.

Causes: A code P0128 may mean that one or more of the following has happened:
* Low engine coolant level
* Leaking or stuck open thermostat
* Faulty cooling fan (running too much)
* Faulty coolant temperature (ECT) sensor
* Faulty intake air temperature (IAT) sensor

Possible Solutions: Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
* Verify coolant strength & level
* Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
* Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
* Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
* If the above items check out good, replace the thermostat
* If Nissan vehicle, check for Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct the P0128 code

Hope helps (remember to rate and comment this).

Check your serpentine belt. If ok then check engine coolant if that is ok take off radiator cap start engine and watch to see if the coolant in the radiator starts to circulate when engine reaches operating temp. If it don't change engine thermostat if all fails to get coolant moving then you will have to change the water pump.

more than likely the thermostat has stuck open. Or opening to soon not allowing the car to reach normal operating temperature. I would start by replacing the thermostat. Clearing the code and running a drive cycle and see if the code comes back. if the code does not return problem solved. if it does return replace the coolant temp sensor.

it has to be either a broken gauge, faulty wiring, or poor coolant flow (although in the summer i doubt the coolant is staying 'too cold' in reality).
First, we will check the temp sensor 1. Pull the temp sensor out and let it hang. 2. Boil water -- it will be at 212* at its boiling point but cooling rapidly, so speed is key. Or better yet, if you have a thermometer and vegetable oil and steady hands, heat that up to 250* 3. Take the water or oil to the sensor and pop the sensor in it. Watch the gauge to see if it reads accurately (should show around 200 if you moved quickly with the water, oil will still be ~240ish and will work better, but is more dangerous). If the gauge is off, then we've found the problem. For better measure, if you have a thermometer you can place that next to the temp sensor to compare results.
if the temp reading is too low (lower than what your thermometer reads), it is also possible that you have a short in the wiring or simply corroded wires connected to it, adding resistance to the wire which would result in a lower reading. (people sometimes use this to their advantage with the air temp sensor -- add a 100ohm or so resistor in-line with the sensor to trick the cpu into thinking the temp is colder, prompting it to adjust the air/gas mix and yielding a couple hp increase, although damaging to your system.)
the more i think about it, the more a faulty wiring feels like it could be the problem. if the temperature is reading accurately, i'd take a serious look at the flow of coolant through the system. if it isn't heating up then it must not be contacting enough heat. there may be blockage somewhere; crimped hose or maybe just mystery gunk.
hopefully this helps, if anything it's a start. most of the time it seems like solutions are found unexpectedly when testing completely different things. good luck!

It could be a faulty thermostat. Did it do the same thing before u replaced the t-stat? It sounds like it's sticking closed, then all of a sudden it opens, coolant flows correctly. I'd consider changing it again. Check to make sure parts store is giving u the correct temperature thermostat also. Maybe it's too high for your Ford, delaying the opening.