I have a 2001 Volvo XC70. It has a trouble code

Well code 128 is for a thermostat problem in the fact the engine is not reaching normal temp. If you have heat the engine is warming up but still may not be getting to normal range. Normal is 190 - 210 degrees.
I would replace the thermostat first and see what happens.

Take it to a qualified repair shop an have it diagnosed properly !

Trouble Code: P0128

Thermostat Rationality Test

Possible Causes:


Check the operation of the thermostat (it may be stuck open).ECT Sensor is contaminated, damaged or it has failed.Inspect for low coolant level or for an incorrect coolant mixture.Trouble Code: P0440

EVAP Purge System Fault

Possible Causes:


EVAP purge valve vacuum supply is leaking or clogged.EVAP purge valve is stuck closed.EVAP purge solenoid has failed.NVLD assembly (leak detection) is damaged or has failed.NVLD switch circuit is open or the NVLD switch has failed.Ground circuit is open.PCM has failed.Trouble Code: P0456

EVAP Small Leak (0.020") Detected

Possible Causes:


.EVAP fuel tank or canister vapor hoses leaking or damaged.EVAP system component leaking, fuel cap loose or missing.Leak detection pump damaged or leaking.Trouble Code: P0622

Generator Field Control Circuit Malfunction

Possible Causes:


Generator field control circuit is open or is shorted to ground.Generator field control circuit is shorted to system power (B+).Generator field ground circuit is open.Generator is damaged or PCM has failed.PCM has failed.Here are the codes and causes.

P0128 means that your engine thermostat is more than likely faulty. The code description is engine operation below thermostat regulating temperature.

code P0128 ECT ( electronic coolant temperature ) engine temperature to low ( replace thermostat ).

code P0300 engine miss fire detected ( replace spark plugs and spark plug wires )

code P0506 IAC ( idle air control ) system idle to low ( clean the IAC valve if problem persists replace the IAC valve )

on my car when that happens i pop out the button on the gear shifter.

Possible problem with the alternator not putting out enough voltage. I had the same problem with my 2002 XC70, putting in a new alternator fixed it. I used a local mechanic who I found on the car talk web-site. Low Cost Volvo parts can be purchased at FCP Groton.

YOU FAULTY COOLANT SENSOR OR PCM.

It is going to be either the temp sensor or the thermostat. replace them both to avoid trouble down the road.