Diagnose Toyota Prius
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toyota prius hybrid drive system

Diagnose Toyota Prius

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Diagnostics on a Toyota Prius is similar to that on any other late model vehicle. If a problem occurs with something that is not part of the hybrid system, it will set diagnostic code that corresponds to the fault and turn on the Check Engine light. But if a fault involves any of the components in the hybrid system (things like the car won't start, it stalls or doesn't run right, battery problems, etc.), diagnostics will likely require a Toyota scan tool (or aftermarket scan tool with similar software capabilities such as the MasterTech), and access to the Toyota service information.

The factory service information is available at https://techinfo.toyota.com for a daily subscription fee of $10, or a monthly subscription fee of $50, or a yearly subscription fee of $350 (this covers access to all Toyota/Scion and/or Lexus makes & models, by the way). The other option is to subscribe to AlldataDIY, Mitchell or Chilton.

Hybrid Warranty Coverage

An important point to keep in mind with regards to diagnosing and repairing hybrid systems is that these vehicles have an extended powertrain warranty of 8 years or 100,000 miles in all states. In California, the hybrid system warranty is 15 years or 150,000 mile, which ever comes first. The warranty covers all of the hybrid powertrain components and high voltage battery.

For more information about California's Zero Emission Vehicle and hybrid warranty requirements, Click Here.

AS long as your Toyota is still under warranty, it can be returned to a Toyota dealer for free repairs.

toyota hybrid synergy drive
Warning: Do NOT touch any orange high voltage cables when working on a Prius.


To access the onboard diagnostics you need a scan tool. Most inexpensive DIY code readers display codes only. Such a tool is better than nothing, but it won't display any sensor values or other system data. For that you need a full-fledged scan tool or scanner software that is up-to-date and can read both generic OBD II P0 codes as well as Toyota factory P1 codes.

Like other vehicle manufacturers, Toyota uses both generic OBD II codes and their own factory OEM diagnostic codes. Almost any scan tool these days will read the generic OBD II codes, but it often requires a more expensive scan tool or scanner software to read all of the Toyota factory codes. Without this information, your diagnostic capabilities will be extremely limited because the factory codes cover faults that may occur in the hybrid system.

Toyota Prius scan tool diagnostic connector

If the Malfunction Indicator Lamp (MIL) is on, you'll want to plug in your scan tool to read the code(s) and any freeze frame data that might have been captured to help you in your diagnosis (if you scan tool or scanner software can display such information). Then, depending on what the code is, you'll want to look at various related PIDs to see what's going on with the sensors and system controls. Problem areas that are going to require some hybrid know-how include any faults related to the high voltage battery or charging system, any faults related to engine cranking, any faults related to the transmission, or any faults related to the hybrid control system.

Like other manufacturers, Toyota provides detailed diagnostic charts for all their fault codes. The charts include a description of the fault, the components involved, wiring diagrams and step-by-step checks that need to be made to isolate the fault.

To see some of the Toyota hybrid fault codes, click on the buttons below:

toyota prius fault code chart

toyota prius fault code chart

toyota prius fault code chart

Toyota Prius Service Precautions:

Toyota Prius Service Precautions

Toyota Prius Service Precautions


For faults that may not set a code, or for specific faults that seem especially troublesome or common, you should check to see if Toyota has issued a technical service bulletin. There are several such bulletins out on the Prius:

* EG021-02 -- 2001-2002 Prius, the MIL may come on and set a DTC P3125, which is an inverter malfunction code. The problem may be caused by a fault in the electronic control unit (ECU) or the hybrid inverter/converter assembly. The bulletin includes a detailed diagnostic chart that takes you through the step-by-step checks that are necessary to figure out what's wrong. The checks include using a scan tool to read the freeze frame data for the vehicle speed that was recorded when the fault occurred (typically between 25 and 44 mph), the acceleration sensor value (in percent), reading the MG2 motor speed (in rpm), doing some math to convert the MG2 recorded rpm to vehicle speed, and comparing the results to see where the error occurred. If the recorded vehicle speed is higher than the converted speed value from the MG2 reading, the ECU is at fault and needs to be replaced.

Like we said, this stuff gets rather complicated so you need to read the entire service bulletin, be familiar with the various hybrid components, and have a scan tool that can read the proper PIDs and freeze frame data.

In this case, if the ECU needs to be replaced, it is located under the passenger side kick panel. Before you touch anything, though, you first need to disconnect the negative battery cable from the 12 volt battery in the trunk (which Toyota calls the "auxiliary" battery). You can now uncover and replace the ECU.

Like most late model cars, if you replace the ECU the computer has to undergo an immobilizer key code registration procedure. On the Prius, you connect a 12 volt battery charger to the 12 volt battery, set the parking brake, place the transmission in park, insert the ignition key and turn the ignition on. Then you leave it on for at least 30 minutes. The computer will recognize the key, sync itself with the key and reset the immobilizer code. After 30 minutes, you can turn the key off and you're done. The Ready light should now come on when the key is inserted and turned on while holding your foot on the brake pedal.

* EG022-04 -- 2001-2003 Prius. The MIL may come on and set a DTC P3130, and maybe also a DTC P3125, which are codes for a power problem or the Ready light not coming on. The fault may occur after the vehicle has been subjected to prolonged driving in heavy stop-and-go traffic, or when it has been driven at sustained high speeds during hot weather. The problem may be an overheated inverter/converter due to loss of coolant or coolant not circulating properly in the inverter/converter cooling system.

Because the inverter/converter is part of the high voltage system, you do not want to touch anything until the high voltage hybrid battery has been disconnected by pulling out the power service plug in the trunk. You then must wait at least five minutes for the high voltage capacitors in the inverter/converter to discharge before proceeding with repairs to the cooling system or inverter/converter assembly. The bulletin covers how to drain, refill and bleed the cooling system, and the replacement of the inverter/converter assembly.

* PG002-06 -- This bulletin covers precautions for removing or installing the inverter/converter assembly on 2001-06 Prius, and 2006 Highlander and 2007 Camry hybrids. Basically, it tells you to disconnect the high voltage hybrid battery before doing any repairs, to wear insulated rubber gloves before touching any high voltage components, to wait 5 minutes after disconnecting the battery on the Prius and Highlander, and 10 minutes after disconnecting the battery on the Camry for the capacitors to discharge, and how to service the components in the inverter/converter cooling system.

* Recall 06V266000 -- A recall of 2001-2002 Prius models for the free replacement of a possible defective crankshaft position sensor that could cause the engine to stall and not restart.

Toyota Prius Diagnostic Tips

High Voltage Leaks! On 2001-2003 Prius, if you see the master warning light on and find a DTC P3009 code, watch out! That's a code for a high voltage leak from the hybrid battery to the vehicle chassis. The battery and high voltage cables are normally isolated from the chassis and other electrical circuits. But corrosion under a cover on the transmission vent can sometimes cause the high voltage cables to short out.

Scan tool not talking to the PCM? If you're working on a 2004 Prius and your scan tool is not communicating properly with the vehicle computer, the ECU may need to be reprogrammed. Toyota issued a recall on approximately 23,800 cars to reflash the computer to correct a programming error. This can be done by a Toyota dealer, or an aftermarket repair facility that has a J-2534 pass-thru reprogramming tool.

If a 2004 Prius suddenly shuts down during rainy weather, the problem may be water leaking past the hood cowl seal. The water drips on the ignition system causing it to misfire. The hybrid software then shuts down the engine and the car won't go. The fix is to dry out the ignition system, and repair the leaky hood seal.

Inverter/converter cooling problems and failures in a Prius are often due to a faulty 12-volt coolant pump. The pump is buried behind the front bumper cover and left headlight assembly (which have to be removed to get at the pump). The part number for the pump is G9020-47020 should you need to replace one (available at your Toyota dealer only). If you have to replace an inverter/converter because it overheated and fried its brains, a new one from Toyota will cost around $3700! So it's important to make sure the cooling system for the inverter/converter is circulating coolant.

Battery cooling problems can cause premature battery failure and turn on the MIL lamp. If the intake filter for the battery cooling housing becomes clogged, it can reduce airflow and cooling for the hybrid battery. Over time, this can cause the battery to run hot and suffer damage. On 2012 to 2014 Prius V hybrid models, battery codes P0A80 or POA7F indicate battery deterioration due to an overheating condition. Check and replace the filter (P/N G92DH-47040), and blow out the cooling housing with compressed air.

Toyota Extends Battery Warranty on 2020 Prius and Other Hybrids

Toyota 2020 Prius and other Hybrids Models

Toyota has extended the battery and hybrid controller/inverter warranty on 2020 model year Prius and other Toyota and Lexus hybrid models from 8 years or 100,000 to 10 years or 150,000 miles, which ever comes first. Vehicles covered include all 2020 and up Toyota Prius models, all other Toyota and Lexus hybrid models, all plug-in hybrid models and all fuel cell electric vehicles.

The 10/150 extended warranty covers the following components:

High Voltage Hybrid Battery

Hybrid Battery Control Module (battery voltage sensor)

Hybrid Control Module (power management control module)

Inverter with Converter

All other hybrid components are covered for 8 years/100,000 miles, whichever comes first.

The warranty applies to the original vehicle owner and all subsequent owners up to the time/mileage limits of the warranty.

To date, Toyota has sole over three million hybrid vehicles in the United States.

Toyota’s 2020 lineup includes seven hybrid models and one fuel cell model:

Fourth-generation Prius with available AWD-e boasts a combined EPA-estimated 50 MPG

The Prius Prime, Toyota’s plug-in Hybrid now has a range of 640 miles on a full charge and a full tank of gas.

First-ever Corolla Hybrid with an EPA-estimated 52 combined MPG

Third-generation Camry Hybrid with an EPA-estimated 52 combined MPG

Second-generation RAV4 Hybrid with an estimated 40 combined MPG

Avalon Hybrid premium sedan with an EPA-estimated 44 combined MPG

New 2020 Highlander Hybrid with an estimated 33 combined MPG

And the current generation Toyota Mirai with an EPA-estimated 67 combined MPGe and 312-mile EPA-estimated driving range.

Lexus hybrid models covered by the new extended 10/150 warranty include:

RX Hybrid with an EPA-estimated 30 combined MPG

UX Hybrid with AWD-e and an estimated 39 combined MPG

NX Hybrid with an EPA-estimated 31 combined MPG

LC Hybrid an EPA-estimated 30 combined MPG

LS Hybrid with an EPA-estimated 28 combined MPG (also available in AWD with an EPA-estimated 26 combined MPG)

ES Hybrid with an EPA-estimated 44 combined MPG

toyota prius diagnosis More Diagnostic Articles:

Toyota Unintended Acceleration: Possible Causes

Common Car Problems

Troubleshooting Random Misfires

Hybrid Safety Hazards

Hybrid Service Tips

Diesel or Hybrid? Which is Best?

Troubleshooting electrical problems

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