Gen III Prius - No Ready - Contactor Stuck Closed Codes
This was a simple diagnosis but an uncommon failure, so I felt it was worth sharing.
I was called to another shop about a Gen III Prius that would not “ready up.” The shop owner told me the customer had the vehicle towed out of the Toyota dealer after they recommended replacement of the HV battery. He also reported the 12-volt battery had just been replaced. I scanned the vehicle for codes and found P0A0D and P0AA1. The P0A0D was set because the shop had pulled the HV battery service disconnect and attempted to start the vehicle. The P0AA1 indicates the module thinks the positive HV contactor is stuck closed. I did a quick evaluation of the HV battery via the scan tool data stream and found it well balanced and 0% delta state of charge. I next reinstalled the service disconnect, cleared the codes and attempted a start – still no ready light and a “service hybrid system” message in the dash. I rescanned the hybrid control module and it now was setting P0AE2, which indicates an issue with the pre charge resistor.
Both high voltage contactors, pre charge relay and pre charge resistor are all part of an assembly called the junction block (part #G92Z047011) under the hybrid battery side cover. This is an update from the previous generations where each of those components were separate parts.
I removed the junction block from the vehicle to be bench tested. Initially it was easier to split the unit in half to expose the bus bars rather than removed the individual components. A simple resistance check with my meter found that on the high voltage side of the positive contactor there was no continuity, which is to be expected with it de-energized. When checking the high voltage side of the negative contactor I found 0.2 Ω resistance, which indicates it’s stuck closed! I removed the contactor from the assembly and confirmed the same readings. Keep in mind, the contactors are just heavy-duty relays that connect the high-voltage battery to the rest of the system when the vehicle is in “Ready.” We’ve all seen relays stuck closed before, but the contactors are safety critical devices that we would expect to be manufactured to a high standard. The majority of hybrid vehicles I work on are Gen II Prius and I have never seen a stuck closed contactor. I’ve seen the codes before, but they were due to a poor ground connection. Also, the pre charge relay and pre charge resistor are used to specifically prevent this.
I sourced a known good used junction block, installed it and all is well. When I returned to the shop to install the junction block, I wanted to get a scope capture of the faulty unit. Truth be told I had a back-probe connection issue and while attempting to start the vehicle several times it readied-up! I hate when this happens! The contactor had unstuck itself. I installed the known good junction block and informed the shop the car is all set. Annoyed the original part started working, I removed the suspect contactor and disassembled it for inspection. The contact points are encased in ceramic. I cracked the ceramic open and found evidence on the right side of the copper contact points being essentially spot welded together.
In summary: I think Toyota tried to save some money when developing the junction block and maybe cut some corners. This unit is also used in the Prius V and Highlander Hybrid. I rarely use resistance testing as definitive diagnosis but as one instructor once taught me: when resistance checking, bad is bad and good is “well, maybe.”
Thanks for reading
Did you take a look at the positive contactor, for evidence of contact arching? Makes you wonder about the pre-charge circuit...
I haven't taken it apart yet. I'm also currently in the process of obtaining and analyzing known good scope captures of the contactor control circuits between the generations to see if there's a difference
Is it possible that a failed or failing 12 volt battery could have caused this?
I've seen bad 12 volt batteries set some crazy codes and cause some crazy symptoms but I can't see how it would cause the contactor to stick closed. Interesting thought though.
Nice write up! Thank you
Our hybrid and I , vehicles use same thing and when they first came out this was a issue because the vehicle was not in a sleep state, very low load, thus to much current through contractor welding the close . BMW ‘s electric / hybrid most likely done at shop replacement of 12 v battery
Hi Robert, looks like you reply got a little jumbled. Can you clarify? Your suggesting the 12 volt battery replacment could have been the cause? Can you expand on that?
…I) the letter i vehicles are the electric cars . We must be assured the they have entered sleep mode before disconnecting the HV INTERLOCK. if not the current flow is to great and will arch the relays closed . This sleep mode can take any where from 10 to 20 minutes. Doors close, trunk,hood etc. the start / stop button internal light will be off,! Than disconnect the HV interlock . If you…
Thanks Robert, that makes sense. Good tech tip. I haven't worked on many of the BMW-i's
Matt, Great write-up! I've always taught techs to use the SAE voltage check method to be safe prior to working on the High Voltage side of system just in case the "never happens" contactors were stuck. You can't let a "lack of time" excuse keep you from working safe around High Voltage. Interesting same unit common across Toyota and BMW lines. Thanks to all especially diag.net for…
Matt great case study, thanks for sharing with us. Documentation/testing is key, imagine if the unit was disturbed without testing and all of a sudden it works! Again, thank you for sharing.