P0128: P0128 Diagnostic Guide

What this code means

P0128 often means the PCM may be seeing the engine take longer than expected to reach normal operating temperature.

What the vehicle may do

The engine may warm up slowly.
The temperature gauge may appear lower than expected or may not match the complaint.
Cabin heat or climate control performance can sometimes be part of the customer concern.

Possible fault areas

Possible coolant level concerns
Possible thermostat operation concerns
Possible cooling system leaks or fluid contamination
Possible sensor feedback concerns

Diagnostic path

Opening context

On this 6.7 Power Stroke Super Duty, P0128 means the PCM may be seeing the engine take too long to reach normal operating temperature. The truck may warm up slowly, the temperature gauge may not act the way the driver expects, and cabin heat or climate control performance can sometimes be part of the complaint. Broadly, you’re thinking about possible coolant level problems, thermostat operation, cooling system leaks or contamination, and sensor feedback. Start with the basic system checks, keep the diagnostic path structured, and do not open the cooling system hot. Let it cool first, because pressure, steam, and hot coolant can come out hard enough to seriously injure you.

Choose the P0128 path

For P0128, the path is the one for an engine that does not reach normal operating temperature. Begin with a careful visual inspection for obvious physical damage or failure in the cooling system area. If you find a concern, repair it as needed. If nothing obvious is found, move on and confirm the code with the scan tool.

Confirm whether P0128 is active

Run the PCM self-test with the scan tool. If P0128 is present, continue to the coolant level check. If P0128 is not present, the cooling system is considered operational at that time. From there, if the complaint is really an inoperative temperature gauge, check gauge operation. If the complaint is climate control related, check that system. And if other codes are present, check what they mean before chasing this cooling path any further.

Coolant level decision

Let the engine cool before checking the degas bottle. Visually check the engine coolant level in the bottle. If the level is within specification, the supported repair path is to install a new thermostat, clear the DTCs, and if the code returns, check for correct CACCT sensor operation. If the coolant level is not within specification, do not jump straight to the thermostat. Move into the low-coolant diagnostic path and find out where the coolant concern is coming from.

Low coolant path and pressure test setup

On the low-coolant path, start again with the basics: inspect for obvious physical damage or failure, then check the coolant level and pressure test the cooling system. Turn the engine OFF, check and adjust the coolant level as necessary, then remove the degas bottle cap only after the system is cool. Inspect the cap and bottle sealing surfaces for problems such as cross-threading, burrs, or a damaged O-ring. If sealing issues are found, or if coolant was expelled through the cap and may have contaminated the gasket, install a new cap and/or degas bottle.

Cap and system pressure test

Test the degas bottle cap with the Pressure Tester and adaptor, Snap-On TA53 or equivalent. The cap must hold a pressure of 145 kPa +/- 21 kPa (21 PSI +/- 3 PSI). If it does not, install a new cap. Then connect the Pressure Tester and adaptor, Snap-On TA52, AST ASSFZ-47, Redline RDL95-0750, or equivalent, to the degas bottle. Press the plunger slowly, because pumping too fast can give a false pressure reading. Bring the system up to between 124 - 138 kPa (18 - 20 PSI), and watch the gauge for approximately 2 minutes. Do not over-pressurize the cooling system, or components may be damaged. The pressure should not drop during that time. If it drops, inspect for leaks and repair as necessary. If no external leak is found but the pressure still drops, inspect the coolant for engine oil and the engine oil for coolant, because the leak may be internal to the engine. If pressure holds, remove the tester and adaptor, then install the degas bottle cap until it contacts the hard stop.

Fluid contamination and internal leak checks

If the system does not show an external leak, keep following the low-coolant path. Inspect the coolant in the degas bottle for transmission fluid. If transmission fluid is present in the coolant, install a new transmission fluid cooler and repair the transmission as necessary. Then check the other direction by inspecting the transmission fluid for coolant. If coolant is present in the transmission fluid, again, install a new transmission fluid cooler and repair the transmission as necessary. Next, check the engine side: inspect the coolant for engine oil, and inspect the engine oil for coolant. If either condition is present, continue with diesel engine diagnosis rather than treating this like a simple thermostat-only problem. Then carry out the EGR cooler leak test. If the EGR cooler leaks, install a new EGR cooler. Finally, use UView® Combustion Leak Tester part number UVU560000-R or equivalent to check the coolant for combustion gases. If combustion gases are present, continue with diesel engine diagnosis. If they are not present, the cooling system is operating correctly at that time.

Thermostat handling

Thermostat replacement is supported only after the applicable diagnostic checks have been carried out. If the thermostat is removed for visual inspection, look for damage like a valve that is not fully seated, foreign material in the main valve, a bent or broken frame, flange, or spring, wax leakage or a bulge at the wax reservoir, other distortion, or a worn O-ring seal. If damage is found, remove any foreign material or broken pieces and install a new thermostat. If no damage is found, do not try to force the thermostat open with hot water or another heat source. That is not an accurate thermostat function test and it can damage the thermostat.

Other numeric checks that may come up

If the work expands into the supporting cooling-system electrical or operation checks, keep those numbers exact. On the block heater power cable circuit check, if any circuit measures more than 5 ohms, install a new power cable. On the block heater itself, if the resistance does not measure between 12.5 and 17.0 ohms, install a new block heater. For the coolant pump operation check, allow the engine to run for 10 minutes. For the thermostat operation check, also allow the engine to run for 10 minutes.

Close and verify

The key on P0128 is to separate a true warm-up problem from a low-coolant or leak problem before condemning parts. Confirm the code, check the coolant level cold, follow the correct branch, and after the supported repair, clear the DTCs and make sure the code stays gone. For more diagnostic training, visit stepdiagnostics.com.