P0A78, P0A94 and P0A7A errors on long trip. 2007 Prius

Those codes point to an inverter problem. If the car had been in a front-end accident, perhaps the wiring harness to the inverter was damaged and not properly repaired.

If the wiring harness and connectors are in good condition then it is likely that the inverter will need to be replaced.

You may wish to take your car to your local Toyota dealer for diagnosis. Good luck.

 

To thehighend:

I too have a salvaged prius with similar problems (bumps, traction light blinking, jerky driving). I also received the p0a78 code. Took to a dealer and they concluded transmission needed to be replaced. Took it to another mechanic for a second opinion and he was sure it wasn't the transmission....so I'm really confused. What I want to know is how did your prius turn out? What was the problem? Hoping I can fix mine!

Thanks,
Aaron

 

Depending on your year and INF (subcodes) the following TSB EG058-06 may apply (replace HV ECU with updated unit, may be under warranty, check with dealer)

 

Thank you for the info. Sorry it has been so long from last post. The prius is a 2008. We actually had the inverter replaced with a used one this past December. Amazingly, all our symptoms disappeared. But then, just this week, the herky jerky and fall safe mode returned. So frustrating. Considering replacing the transmission. But I will definitely show this document to my mechanic. Thanks again.

 

I suggest that you get Mini VCI so that you can obtain the diagnostic trouble codes logged by your car. It is premature to be talking about replacing a labor-intensive part like the transaxle until you know what the DTC are.

 

Don't forget to check the inverter coolant pump, or better yet check coolant temperatures with Techstream / Mini-VCI until you've been driving 20 minutes or so. If the temperatures are high the pump may be failing.

 

What is considered high? I have engine link showing converter temp of about 140F.
I replaced the HV control ECU last night. Seems better. Haven't gotten the P0A94 code back yet. Just have the P0A78/282

 

Did you replace the HV ECU with the newer revision part number as shown in the TSB?

Check the tightness of the inverter wiring harness connections beneath the inverter cover. If they are OK, then the P0A78 code indicates the inverter needs to be replaced (again).

This is the first post where you've mentioned P0A94. Had that code been previously logged by your car? If so, that indicates a problem with the DC/DC converter within the inverter.

 

Yes, I often get that p0a94 code....here is a detailed summary of what I've had done so far...BTW, thank you for taking a look!


September 2014:

Codes (with INF codes in parentheses)

P0A78 (1=506, 4=282, 5=287): Drive Motor A Inverter Performance (possible trouble areas = inverter assembly, wire harness/connector, HV transaxle, hybrid motor, HV ECU)

P0A94 (2=556, 3=555): DC/DC Converter Performance (possible trouble areas = wire harness/connector, HV transaxle, hybrid motor, hybrid generator, HV ECU, inverter assembly)

C2300 Electronic shift lever (ACT system) Malfunction (possible trouble areas = shift control actuator, transaxle park lock control, trans ECU, wire harness/connector)

C2318 Shift Lever Low Voltage (possible trouble areas = HEV fuse, IGCT relay, wire harness/connector, auxillary battery)

Tests/Action: Multiple test drives, experienced symptom. Replaced 12 volt battery. Tested transmission/motor-generator with Megger, passed test (note-dealer said it didn’t pass for them). Inspected and cleaned both front tone rings and wheel speed sensors. Noted crushed shielding to a sensor wire. Inspected ground connections and cleaned a bad ground. Extensive test driving, symptom returned, but no codes returned.


Recommended drain/refill transmission. Customer declined.



November/December 2014:

Customer symptom getting more frequent and worse.

Codes

P0A78 (282, 287) see above

P0A7A (548, 554, 555): Generator Inverter Performance (possible causes = wire harness, inverter)

P0A94 see above

P0A40: Drive Motor A Position Sensor (possible causes = wire harness, hybrid motor, HV ECU)

Tests/Action:

Too intermittent...difficult to diagnose a specific cause. Inspected numerous connectors. Test drove extensively. Scanned and monitored vehicle data. We know computer is perceiving high voltage spikes. Inverter brackets are bent. Replaced inverter with used unit.



April 2015:

Customer symptom disappeared for awhile after inverter replaced, including during a long road trip. But now symptom has returned.

Codes

P0A78 (282) see above

P0A94 (548) see above

Freeze frame showed that both codes were recorded at the exact same moment, but the P0A78 code showed the power supply voltage to be 11.75 and the P0A94 code showed 13.76 volts. This is very important clue which indicates problem in 12 volt system. This led us directly into inspecting the power supply to the hybrid ECM (see below)

Tests/action: Examined wiring diagrams for power supplies on low voltage circuit, battery, IGCT relay, ignition switch. Battery is good. Suspect IGCT relay. Accessed relay block and when removed it for inspection, discovered the power supply was only loosely plugged in. Plugged it in fully. Loose connection could explain the difference in the supply voltage in the freeze frame data.


Customer reports vehicle still has symptom.

 

Well, my prius is a 2008, so I'm not sure if that still applies. I had called the dealer and asked for the part number to the HV control ECU.

 

I assume your car was in a front-end accident? Has the engine wiring harness been throughly inspected for damage? Since the inverter was hit (as evidenced by mention of bent brackets below) the wiring to the inverter may have been stressed and damaged.

I would drain and refill the transaxle ATF just as a matter of performing good maintenance, if the car has logged more than 60K miles.

There may be some other intermittent/loose connections. Again, remove the inverter cover and inspect the wiring harness connectors within to make sure they are tight and that the wires show no damage.

Intermittent electrical problems are challenging to resolve, and Prius, heavily laden with complex electronics, is even more difficult than most. Good luck.

 

I was getting P0A94 code i changed the inverter they it went off. Then i got again the same code i bought a new ECM & installed it but now my car is not starting all lights turns on but not starting the engine. It does start on my old ECM but not the new one any help please.

 

Hello and welcome to Priuschat!

If you have a P0A94 code for DC-DC converter performance, it might help if you can list the INF or detail code (it's shown in the freezeframe data). That way ee can give better advice about how to find your problem.

If you replaced the Hybrid Control ecu, then you need to jumper pins TC & CG of the OBD2 connector and leave the ignition on for 30 minutes to "program" it (perform a "handshake" initialization between HC and certification ecu's?) . Use a charger so the 12V battery doesn't go too low.

See the service manual (under immobilizer - registration) for details.

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