Hello so I’ve been getting my cel on and it’s been throwing me a p0128 code I’ve been trying to figure this out but I can’t so far I’ve changed the MAF, Coolant sensor and the thermostat and it still comes on. I have a couple of things done to the RDX I have a K&N intake, rv6 catless, downpipe atlp exhaust, DW fuel injectors, dw fuel pump, rywire harness, hondata flashpro on a custom tune, running e85 please let me know if you guys can help!

 

the code is specific for a thermostat...what brand thermostat did you replace with?
also, there might be a blockage somewhere.

is there a leak? check

 

Where is your temp gauge when the car is at operating temp?

 

Also, which coolant temp sensor did you replace?

 

Acura RDX P0128: Coolant Thermostat – Below Temp

P0128 is a relatively common OBD-II trouble code. It is triggered when your Acura RDX‘s coolant temperature fails to reach the proper operating temperature.

It’s a generic power-train code, which means that regardless of who manufactured the vehicle, it’ll have the same meaning (1996+).

RDX P0128 Symptoms

There aren’t many symptoms with P0128, unless it gets cold. Here are the symptoms associated with this trouble code:

• Lack of heat: The biggest symptom of the P0128 trouble code in the Acura RDX is going to be lack of heat. When the thermostat doesn’t bring the engine all of the way up to the proper operating temperature, it means that the water going through the heater core isn’t as hot. This leads to less/no heat.
  
• Service Engine Soon: other than the heater not working as good, the only other symptom of P0128 is typically the service engine soon light itself.

P0128 Causes: Acura RDX

Fixing P0128 is usually a pretty easy thing to do, since 90% of the time it’s going to be the thermostat that is throwing the code. Here are the most common causes of P0128:

• Thermostat Stuck Open: far and away, the most typical cause for the P0128 code is going to be a thermostat that is stuck open. When a thermostat is stuck open, it allows coolant to flow through the engine at all times. When this happens, the engine is “over-cooled” and never reaches the proper operating temperature.
  
• Wiring: if the thermostat itself isn’t the problem, it’ll be the wiring/sensor associated with engine water temp detection, or the coolant temperature sensor itself.

Acura RDX P0128 Diagnosis

A good way to tell if it’s a sensor issue or an issue with the thermostat is going to be whether or not the engine is running at its proper operating temperature or not. It can be really difficult to tell this in a vehicle that doesn’t have a temperature gauge.

A good way to at least get a good idea as to whether or not it’s the thermostat or the sensor is to start the car cold and let it idle for ten or so minutes. If you feel little to no heat from the heater, then it’s a pretty good indication that the thermostat is the problem, and not a wiring issue.

Also, a mechanic will typically check under the hood where the radiator hose meets the thermostat housing. If it is easy to pinch the hose, typically water will be flowing through it. If you pinch your hose make sure that you are aware of the belted accessories and the fan. The fan can come on at any time.

Conclusion

P0128 is not a serious threat to the immediate drive-ability of your RDX. It should be noted that most modern fuel injected engines to like to run at 200+ degrees. Letting them run too cool can problems after an extended period of time.

 

Are you able to monitor your exact coolant temps while driving? That should give you an idea whether your thermostat or something else is the issue here. I don't mean the little bars on the gauge cluster, but rather get an OBD reader and see the exact temp being reported by the ecu. In 1G RDX, I believe the thermostat starts opening around 170F / 78C, and your normal operating range will be in the 180-200 range (it will fluctuate). If it's constantly 170 or lower, then your thermostat is likely stuck open. If the coolant temp sensor itself is an issue, you'll probably notice right away when trying to read it on the scanner.

 

There are 2 sensors on the RDX. One on the block and one on the bottom of the radiator. I believe the one on the radiator is used for the temp gauge and the one in the block goes to the ECU. I'm pretty certain that a fault in either or both will trigger a p0128. Since he just replaced the thermostat and 1 of the sensors, I would lean toward the remaining sensor. It's definitely possible a new thermostat could be bad, but more likely it's an old sensor.

-Matt