I have a code P0128, what is my problem.

This code is for your Coolant Thermostat - It's detected that the Coolant Temperature is below the Thermostat Regulating Temperature

This code may mean one or more of the following has occurred:

• Low engine coolant level
• Leaking or stuck open thermostat
• Cooling fan running too much
• Faulty coolant temperature sensor (ECT)
• Faulty intake air temperature sensor (IAT)

Check the following to see if it resolves your problem:

• verify coolant strength and level
• verify cooling fan operation and replace if necessary
  
• verify ECT sensor operation and replace if necessary
• verify IAT sensor operation and replace if necessary
• if all above checks out, replace the thermostat
  

P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature).

Normally this code used to indicate a faulty thermostat.
However there are several other possible causes for this.
they include,

• Low engine coolant level
• Faulty cooling fan (running too much)
• Faulty coolant temperature (ECT) sensor
• Faulty intake air temperature (IAT) sensor

You should first check your coolant level. It would be a good idea to flush the coolant system then refill with coolant if this has not been done before. Before you refill with fresh coolant you should consider replacing the thermostat now. It is a relatively easy item to replace with basic tools and moderate mechanical skill. The cost should be between $15 to $30. A repair manual is a good investment and will give detailed instructions on replacing the thermostat.

After this you should have the codes reset.

If the code comes back you may need to replace the coolant temp sensor however the coolant flush and thermostat usually takes care of the problem.
Hope this helps.