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Home >   How To >   How to Repair AC Parts >   Known Issue: Toyota Prius AC Oil Contamination

Known Issue: Toyota Prius AC Oil Contamination

Proper Knowledge of Air-Conditioning Systems is Vital!


Hybrid A/C systems may be classified as service-critical from more than just one perspective. Service-critical in the sense of higher demands to the required know-how and skills than for any other auto air-conditioning system. Working on vehicles with high voltage drive systems also requires know-how on how these systems work and how they should be treated. For instance, working on air-conditioning systems with an electrical compressor requires knowledge on how to shut down the high voltage system safely and correctly. These high voltage systems are not generic but different for each individual vehicle type.


Another aspect is the accuracy of working on these systems. Auto air-conditioning systems with an electrical aircon compressor are equipped with a special compressor oil. In most cases, this concerns a special ester oil with special additives. They offer maximum protection against high voltage being passed through the compressor body. If another universal ester or PAG oil is used instead of this special oil, there's a risk that the aircon system is not grounded correctly. A mechanic who is unaware of this and touches the aircon compressor at the wrong moment, runs the risk of getting an electric shock.


In case of oil contamination, Toyota leaves no room for doubt:


** "Electrical insulation performance may decrease significantly if even a small amount of oil other than ND OIL11 is used (or enters) in the refrigeration cycle, causing the DTC to be output. If other oil is accidentally used and a DTC is output, collect the compressor oil in the compressor and replace it with ND OIL11 to increase the ND OIL11 ratio amount. Replace the main components (evaporator, condenser, and compressor), if a large amount of oil other than ND OIL11 enters the system. Failing to do so may cause electrical insulation performance to remain low, causing the DTC to be output".


The Toyota procedure leads to two questions. The first one is what Toyota exactly defines as "a large amount of oil"; not unimportant to know. Replacing the aircon system's evaporator, condenser and compressor is a costly measure which will not please the customer, to say the least. For sure, it will involve a full day of work, plus buying the required components.


Well now, the official lubricant filling quantity of the Prius II is approximately 100 ml. Other official technical documentation by Toyota quotes that as much as 1% of oil contamination may have a serious impact on the oil's capacity to insulate from passing voltage. In other words, according to Toyota already the mixing of 5 ml of foreign oil may be reason for Toyota to recommend replacing a substantial part of the a/c system. This goes very far and seems hard to sell to the customer. Now the question is, would Toyota apply this rule as strictly as they put it to paper?


This seems unlikely. Even Toyota may have a tough time selling such a costly repair to its customers. Many customers may react appalled and disappointed. Other customers may decide to leave the a/c system unrepaired. For sure, this is not good publicity for the brand. Maybe the real level of contamination tolerance is higher than 2%, in contrast to what is officially indicated in the service guidebook, and only to make sure that Toyota workshop mechanics respect safety procedures under all circumstances and won't even think of working on the a/c systems without protective safety gloves.




Whatever the real reason may be, how should one best act if a Prius II enters the workshop and the quality of the oil needs to be checked?


In such a case, it is important to know that the electronic system in the Prius is continuously monitoring the ground isolation of the system. If the measured resistance is too low, an error code is logged. If the data bus diagnostic system diagnoses low resistivity, error code POAG-611 will be indicated. Simultaneously the a/c system will be disabled. However, the high voltage system will not be shut down. The latter is something which often fools service mechanics who think that such an error code will surely shut down the entire system. This is not the case; A/C system yes, high voltage system no.


It is also of importance to know that error code POAG-611 will be triggered by other issues. A defective electric compressor motor is one of them. The other one is a disturbance in the CAN system. One option to find out whether code POAG-611 is caused by oil contamination is to investigate the vehicle's service history. A second option is by eliminating the occurrence of the other possible causes. By the way, one of the possible causes for the compressor to fail is the use of the wrong oil.


Checking the Compressor:


One of the ways to check the compressor motor is as follows:


• Erase all error codes and switch the ignition on.


• Engage the aircon system, with blower and cooling on maximum setting.


• Switch the ignition off again after three minutes and remove the service plug from the high voltage system according to official procedure.


• Disconnect the two-wire connectors from the high-voltage compressor and connect a grounded ohmmeter to each of the two terminals on the motor (111. 1). Each should have ultra-high resistance (2 megaohm or more). If the measured values are lower, this indicates a shorted motor and the compressor needs to be replaced. After replacement the test needs to be done a second time. The procedure is as follows:


• Erase all error codes and switch the vehicle ignition on.


• Now engage the A/C system and set the blower speed to LOW and the temperature setting to approx. 25°C.


• Run the A/C system for approx. 10 minutes, so oil can collect inside the compressor.


• Disengage the a/c system and leave the vehicle sitting overnight.


• The day after, disconnect the high-voltage system and repeat the compressor resistance test. In this case, the measured resistance should be 3 megaohm or more. If the measured values are also too low now as well, it may be assumed that the oil is wrong.


Contaminated Oil:


Now there you are. You have determined that the oil is contaminated. What now? Does it mean that you have to replace the entire system, as prescribed by Toyota's official procedure? The question still is whether Toyota dealers also follow this rule so strictly, especially when a customer refuses to pay € 2000-3000 for such a problem. By the way, Toyota does seem to keep an opening here with its statement, as mentioned in **.


One of the options is to recover as much oil as possible from the a/c system and refill the a/c system with the correct oil afterwards. The worst that can happen is that this leads to no result, still leaving the customer with the option to decide for the expensive repair. Here are some tips how to do this:


1) Remove as much oil as possible from the system and refill it with the same amount of ND-11. Now erase the error codes* and start the system anew. If no more error codes occur, the problem seems to be solved. If you want to be sure to remove as much oil as possible from the system, leave the aircon system running for at least 10 minutes to collect as much oil as possible inside the compressor. The electric compressors which are used in Lexus and Prius models have been constructed such that a maximum of oil is held inside the compressor. Letting the compressor run, therefore makes a lot of sense.


2) If the system is badly contaminated and does not engage anymore, flushing the system may be an option. No simple option but, again, certainly worth trying if one considers the cost for replacing the compressor, condenser and evaporator. Success is not guaranteed here as well, especially if it involves a system with high-efficiency heat exchangers with micro-channels.


Good Communication with the Customer!


If all these options do not lead to a solution, the customer has the only option left and that's to decide for replacement. It is important to involve the customer from the very beginning and contemplate all the various options how to solve this problem. So, inform him in detail about the official Toyota procedure, its background, what the possible alternatives are, and - especially- that these alternatives are no guarantee for success.


Note: Please be aware that error codes in most Toyota/Lexus hybrids only occur in the first seconds of operation.


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