A combination of failures in 2007 Toyota Prius leads to longer test

Our customers recently brought in their 2007 Toyota Prius because the check-engine light was on. We asked them if the car had any symptoms; they stated it did not.

Once we got the car into the shop, we connected the Toyota Techstream diagnostic tester and pulled codes. There were two codes stored: ECM P2601 (coolant pump circuit range performance) and P1116 (coolant temperature sensor circuit stack for coolant heat storage system). We then performed a test of the coolant storage system. During the test, we concluded that we must replace the heater coolant control valve first.

Now, before we get started, let’s talk a little more about the multiple cooling systems on the 2007 Toyota Prius. This car has two cooling systems: one for the engine and one for the (inverter) hybrid system. The engine cooling system maintains the engine temperature and delivers hot water to a reservoir for heating the cabin. It does this with the use of two water pumps and a heater control valve. The engine has its own mechanical water pump, and the heater (HVAC system) has an electrical water pump. Once the engine is running, hot water flows to the heater control valve, radiator and then back to the engine. The invert or hybrid cooling system is a separate system. The inverter used an electric water pump to regulate the temperature of the inverter.

Diagnosing the problem

After performing the test plan on the two codes, we concluded that we had to replace the engine heater control valve first. This valve delivers hot coolant from the engine to a heater reservoir to the cabin. It seemed as if the valve was not opening all the way and causing a restriction of hot coolant to the reservoir. We explained to the customer that the heater valve must be replaced first, but the heater circuit delivery pump may still have a problem. We then replaced the heater valve and test-drove the car. During the test-drive, all of the monitors passed and we sent the car on its way.

Sixteen days and 259 miles later, the check-engine light was on again. Code P2601 (coolant pump circuit range performance) had come back. This was not a total shock, however, as we had explained to the customer this might happen. The only reason we did not replace the heater water pump last time was that it was possible that the heater valve may have been the only problem.

We replaced the heater water pump and test-drove the car again. After the test-drive, all of the codes cleared and all of the monitors passed their tests. We then let the car go again thinking that all was OK.

Twelve days and 130 miles later, we got another call from the customer that the check-engine light was on again. The car was delivered to the shop, and we pulled a code. This time it was a completely different one: P0A93 code (inverter cooling system performance).

After performing a visual inspection, we could see the inverter coolant reservoir was slightly low. It was possible that the inverter cooling system had an air pocket. We bled the inverter cooling system and tested it again. The code came back after 20 miles of driving. We then disconnected the inverter coolant pump and checked the connector for power and ground. It was good. We then put an alternative power supply on the invert pump and checked it. The inverter coolant pump was inoperative. We then replaced the inverter coolant pump. We next cleared codes and test-drove the car three days and 126 miles; we weren’t going to take a chance that a new code would pop up.

We work on Toyota Prius models all the time and have a tremendous amount of experience with them. Yet we had never seen this combination of failures before. Because of this experience, we will always replace the heater coolant pump and the heater control valve together if we see both the P2601 and P1116 codes together.

Seeing the P0A93 codes so close to the other was also unusual for us. I would have to deduce that a restriction in the heater control valve may have raised the temperature in the engine bay. Either way, when we see codes like this again, we will take the cars on much longer post-test-drives.

Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call (650) 960-6988, email [email protected] or visit sunnyvaleforeigncar.com.

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