How to Diagnose Hybrid Vehicles

How to Diagnose Hybrid Vehicles

This article is part two in our series about the electrification of the auto industry. We’re talking about electric vehicles, their history, how they work, how to service them, and what the trend towards more electric vehicles means for our industry. Click here for the first article in this series.

Diagnosing Hybrids

Working on a hybrid vehicle can range anywhere from confusing to plain dangerous if you don’t know what you’re doing. It’s important to have a basic understanding of the vehicle you’re working on as well as the tools you need to keep yourself safe. We touched on this topic in the last article, “Hybrid Vehicle Safety and Basics.” Here we are diving into the most common mechanical failures with these vehicles.


One of the most common failures in hybrids, likely no surprise to anyone reading this, is the battery. Although, it may not be the battery you’re thinking. The lowly 12v battery is the guy that has to start the party from the word “Go.” It’s the 12v battery’s job to wake up the hybrid vehicle ECU and shut the hybrid battery contactors. It’s very easy to overlook the health of the 12v battery since you don’t get the typical “slow crank” complaint of a dying battery in a non-hybrid vehicle. It works perfectly fine until it doesn’t.

The hybrid vehicle ECU has a lot of thinking to do right away when it wakes up, and it’s very picky about the order and speed with which the hybrid battery contactors operate. This is a big job for the 12v battery, which is intentionally small to keep the vehicle from being too heavy. If you find yourself chasing odd contactor codes, or you have a Prius that won’t “Ready On” properly, which is the Prius version of KOER, make sure the 12v battery connections are clean and tight and the 12v battery is in good shape.

Car Battery

Another quick note on 12v batteries—most of the time these are not the usual flooded acid batteries you may be used to. You’re more likely dealing with an Absorbed Glass Mat battery or AGM battery. Hybrid manufacturers (Toyota mainly) are very fond of putting the 12v battery inside the car. Why? No idea. AGM batteries don’t off-gas as badly as flooded batteries—if you find a battery mounted inside the car, odds are it’s AGM. AGM batteries can’t handle the aggressive charging that normal batteries can and you have to use a special charger set to AGM mode, or you’ll cook the expensive battery. If you don’t have an AGM mode, then just go low and slow with your charger.

More Batteries

The battery you imagined is most likely the hybrid battery. And no matter the vehicle, these batteries wear out from a single day of use. The most typical battery you’ll find is Nickel Metal Hydride or NiMH. These batteries are all put together in series to boost the voltage. Here is a look at a Prius Battery module—you can see that it has 6 cells, at a theoretical 0% State of Charge (SOC) the cell has 1V, at a theoretical 100% SOC the cell has 1.5V. You will realistically never see them hang around near 1V, and under regenerative braking, they can sometimes exceed the 1.5V for a short period of time. This module is used for Prius Gen 2 and 3. Gen 1 is very similar.

Hybrid Battery Cell

Here is an example of a battery module from a Honda Civic Hybrid. Honda went with the stick instead of the prismatic rectangle, but it functions very nearly the same.

Hybrid Batter Module

Note that two sticks are present in this picture and each stick has 6 cells.

In the Data list, you’ll see the terminology “Battery Block” for Toyota, and “Battery Module” for Honda. This is the voltage reading that the battery ECU is seeing from two of these rectangular or stick modules in series. Let’s do the math. Two modules in series equal 12 cells. Cells have a total range of 1-1.5v, so the data should show a total range of 12-18v. You’ll also see SOC, or State of Charge, and Total Battery Voltage. These are all good things to watch when you have a DTC for a battery issue, maybe a P0A7F – Hybrid Battery Deteriorated. There are some additional tools out there that can help you with this as well.

There is no “known good” voltage for these battery block PIDs. Suffice it to say that the hybrid vehicle ECU doesn’t want to get to 0% or to 100% charge, and that’s not healthy for the battery. Instead, it wants to maintain between 40-70% as best it can, with a real range of 20-80%. The important thing is that your battery blocks do not wander away from one another. If you see one block is at 13v and another is at 14v, that’s a problem. Electrons are not flowing as well through one block as the other. The range of difference that is tolerable depends on the model, but around .2v is fine if you’re beating the battery up—anything more than .5v difference is an issue.

Check out this link for a battery stress test that was developed by AR&D. It’s a good real-world test to see how your hybrid battery is doing.

Check out some of the data PIDs below.


If you have some modules wandering away from one another during testing, then either the battery is genuinely failing (most likely) or the module that watches the battery voltage is drunk at the wheel (less likely). In many cases, by the time you get your hands on a vehicle, someone else has already worked on it, and they’ve hung a battery replacement on it along with a nice warning sticker that says “Do not open case, warranty is void if seal is broken.” If you see one of these, stop immediately, assess the nature of the warranty and then get a new battery from whoever owes it to the customer.

If the battery has not been replaced and data stream shows wandering voltage, then it’s time to dig into the battery case. On a Prius, there are several visual clues that can lead to the replacement of the battery. Disable the high voltage circuit, then take the top of the battery case off and look at all the battery blocks—if you see any that are weeping a clear or blue liquid, this is electrolyte from the battery cells. The electrolyte in a Prius battery is COH (Potassium Hydroxide) and when it leaks out, it’s as conductive as a pool of liquid copper, so be careful when you see it.

Hybrid Battery Block

Another element to remember is the “bus bar,” the long row of washers and nuts in the orange case. Keep an eye on the negative terminal for corrosion—this is pandemic for Priuses.

If the battery pack is OK via visual inspection, you may need to confirm the voltage difference with a voltmeter (see the “Hybrid Vehicle Safety and Basics” article for meter specs). Be very careful when working with a live battery exposed for testing.

When you confirm a bad battery, there are several options available to you. First is you can go OE and get a battery from the dealer, which is not a cheap option. You will, however, get the highest quality battery that money can buy. Second, you can buy an after-market remanufactured battery. This option is likely to save you some money but at the cost of some quality. The outfits that “rebuild” these batteries are trying to match and exercise used battery modules to create a good battery unit. This is hard to do.

The third option: try to rebuild it yourself. See all the arguments about why the after-market cost is lower. Rebuilding a battery is no small feat. At the end of it, you must be comfortable that all the battery modules have the same amp hours, the same voltage per SOC, the same charge/discharge rate, and relatively the same amount of dendrites (clumping of electrolyte) as their partners. This is done by charging and discharging battery modules one by one with DC chargers, monitoring the temp, voltage, and amperage it takes on the way up and the way down. Over and over, graphing it all, compiling it and deciding which modules must go and which can stay. It’s a lot of work and it might make for a fun hobby, but It takes a long, long time to do it even close to properly. There are machines that will do it for you, though they don’t seem to have much luck. If you plan on specializing in rebuilding batteries, then it’s up to you, let us know how it goes!

One last note about hybrid batteries. Most OEs insist they cool by air. Not very efficient, and it’s certainly less efficient when there is dog hair, blankets, or McDonald’s wrappers clogging the vents that the battery uses to breathe. Check out this clean vs dirty vent for the battery in this rear seat bolster courtesy of Toyota.

Rear Seat Vent

Here is a vent on an ‘04 Civic Hybrid, courtesy of Motor Authority.

Civic Hybrid Air Vent

Isolation Faults

This is an unfortunate but common failure among hybrid vehicles. Here’s a diagram of the hybrid drivetrain again.

Hybrid Wiring

Hybrids are designed so that all the “High Voltage” is in its own closed circuit, totally separate from the 12v circuit, totally separate from the vehicle chassis. The two cables leaving the high voltage battery are the battery positive and battery negative cables. They’re heavily insulated from chassis ground. The Inverter, the 3 phase cables going to the hybrid motor(s), and the motors themselves are all very heavily insulated from chassis ground. This is to protect the occupants of the vehicle, and the public in general, from electric shock. There is a minimum specification for this resistance to chassis: it must be at least 500 ohms per 1 volt of high voltage. For example, on a system that uses 300 volts, the spec would be 150,000 ohms minimum. Realistically, most OEs insulate much more than is needed, and the “Isolation Fault” codes tend to set long before it reaches this minimum limit.

When this insulation breaks down, it’s called an “Isolation Fault.” The Prius will try to help locate the fault with a “SubCode” along with a DTC.  The “SubCode” may be called an “INF” code, or “Information Code.” For example, P0AA6 is the generic fault, Hybrid Battery Voltage System Isolation Fault. But a P0AA6-613 is a fault that states “insulation resistance of HV transaxle or motor and generator inverters is low.” The hybrid vehicle ECU has determined that a breakdown of insulation exists in the Inverter, the cables from the Inverter to the transmission, or inside the transmission itself. Here is a lack-luster chart that Toyota uses to try to help.

Hybrid Inverter Diagram

The hybrid vehicle ECU is usually pretty good about narrowing down where the issue is, but be sure to use your insulation tester to verify. As a reminder, here’s what that piece of equipment looks like.

Hybrid Vehicle Insulation Tester

This model has several ranges of voltage it can use for testing. You must select between 50, 100, 250, 500, and 1000 volts. If there is no explicit range requested in the DTC troubleshooting, you want to pick the range that is not less than the system voltage you are working on. For example, the 05 Civic Hybrid has a system voltage of 144v, so you would pick 250v. The ‘03 Prius has a system voltage of 274v, so you would pick the 500v range on the meter. Be sure to read the DTC troubleshooting carefully for the desired range.

When you’re ready to test, for example, high voltage system disabled and high voltage connections exposed, put the Red insulation test lead on the component you are checking, and the Black lead to chassis ground. Take a deep breath and punch the big “Insulation Test” button on the meter. The meter will think about it for a second, and then spit out a number. Typically, you’re looking for the meter to max out on the high voltage battery, and the high voltage cables—this is about 550 M Ohms. Control devices like the Inverter are usually 2.9 M ohms. Motor/Generator stators have a resistance that will ramp up and the faster it ramps up to 550 M Ohms the better. If the resistance bounces up and down over and over, then that Motor/Generator stator is likely breaking down insulation to chassis.

If you find a reading that is low, then it’s time to start isolating the system. Remove the cables from the Inverter for example and test again on the mounts of the Inverter where the cables used to go. If the resistance is still low, then the Inverter has failed.


Now Start Diagnosing Some Hybrids

That covers many of the failures you’ll see on hybrids, and it might seem slightly confusing until you start putting a hand on the vehicle. Practice makes perfect, so collect your own set of “Known Typical” (I avoided Known-Good on purpose) data and resistance readings when time permits. The more comfortable you get with the tools and with the vehicles, the easier they’ll be to work with. Who knows, you might even decide to specialize! Like always, if you ever need a hand troubleshooting a vehicle, please give us a call on the hotline!


Download this battery stress test for testing the conditions of Gen 1-3 Priuses and Gen 1-2 Civic Hybrids