Serving hybrid owners

Jan. 1, 2020
Hybrid vehicles are an excellent choice for anyone looking to save money at the gas pump while helping the environment. Yet worries about expensive upkeep, ranging from high voltage (HV) battery replacement to the fear of dealer-only repairs, weighs

Learning valuable lessons on the top five hybrid service complaints.

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Hybrid vehicles are an excellent choice for anyone looking to save money at the gas pump while helping the environment. Yet worries about expensive upkeep — ranging from high voltage (HV) battery replacement to the fear of dealer-only repairs — weighs heavily on the minds of many hybrid buyers.

While automakers have sought to ease these concerns by offering long warranty periods on hybrid components (ranging from eight years/80,000 miles for Honda's hybrid vehicles to eight years/100,000 miles for Toyota, Lexus and Ford hybrids), catastrophic drivetrain failures aren't the only repairs hybrid owners must fear.

Though most of today's hybrids run on the same internal combustion engine (ICE) as their non-hybrid counterparts, regular maintenance and a few hybrid particular repair items might throw some unwary owners and technicians curveballs.

12-Volt Battery Failure or Discharge

The 12-volt automotive battery is a standard maintenance item that needs tending to about every four to five years on a normal vehicle. Simply match the series size and cold cranking amp rating and away you go to the parts store. When it comes to hybrids, things are a little different — sometimes.

The 12-volt battery on a hybrid vehicle is subject to the same concerns as all other batteries in terms of service life. Misuse or abuse such as extended periods in a discharged state, significant discharge cycling (dead battery) or more normally a simple loss of efficiency over time eventually causes the battery to degrade and fail.

The 12-volt battery runs only the computer and other electronic devices when you turn on the hybrid vehicle. Because an HV battery starts the engine, 12-volt batteries usually are rated pretty low in cold cranking amps, so they can go bad long before they ordinarily would cause a no-start. That's why some hybrid aficionados advocate preventive replacement of the 12-volt battery.

On most hybrids, a traditional alternator is not fitted to the engine. A DC-to-DC converter is used to charge and maintain the 12-volt battery and gets its power from the HV battery. As a result of this connection, a bad 12-volt battery not only can drag down the 12-volt system but can result in DC-to-DC converter overheating and failure.

The second problem revolves around just what type of 12-volt battery to use in what vehicle. Most hybrids use wet cell (flooded) lead acid automotive batteries that are mounted in the conventional front-of-engine compartment location. These batteries are serviced and maintained like any other automotive battery.

Some newer hybrid models have switched battery positions and are now mounting batteries in the rear of the vehicle. This is where the issue gets confusing. If you have a rear-mounted battery, it can be a traditional wet cell (flooded) battery or a new Absorbed Glass Mat (AGM) battery that requires special consideration.

AGM batteries require a charging voltage that does not exceed 14 volts at normal temperatures and even lower voltages when summer temperatures soar. Unfortunately, almost all older automotive chargers have a permanently fixed set point voltage that exceeds 14 volts, and this spells trouble. Subjecting the batteries to 14.6 volts for a prolonged period eventually will destroy them.

A good rule of thumb is to use an AGM approved battery charger for any battery mounted in the rear.

Dirty Throttle Body/No-Start

According to hybrid repair specialist David Taylor of Re-Involt Technologies in Sanford, N.C., "This is such a common problem with the first generation Prius, that we check this first when faced with a no-start condition."

The reason for the no-start condition is that the Prius engine control looks at Mass Airflow Sensor (MAF) signal while cranking over the ICE during cold starts. If the hot wire in the MAF is dirty or if the throttle plate in the throttle body is coked (dirty) and sticks, the engine will not start.

There are two problems here: one of problematic design and the other of incorrect maintenance.

Because the ICE on the first generation Prius uses a more efficient Atkinson cycle, the intake valve is held open longer than normal to allow a reverse flow of intake air into the intake manifold. If there is anything amiss in the engine that would allow oil to get into the intake tract, such as a faulty PCV valve, the engine will suck the oil back in with the reverse flow of air and eventually contaminate the MAF hot wire and along with heat will cause the throttle body coking.

But what if all is functioning properly? This condition also can be caused by unwary or unknowing consumers or technicians who apply incorrect maintenance procedures when changing the oil. A careful examination of the oil capacity specifications will show that the Prius engine requires no more than 3.7 quarts of engine oil. If 4 quarts of oil are poured into the engine, this creates on overfill condition. Oil is pushed back into the intake and circulates via the PCV system causing the no-start condition.

Poor Fuel Economy Complaints

Hybrid vehicle owners particularly are interested in fuel mileage, and therefore are concerned any time they see what they believe to be a reduction in fuel economy.

Hybrid expert Craig Van Batenburg of the Automotive Career Development Center (ACDC) suggests checking the simple stuff first. Are the wrong tires installed on the vehicle? Hybrids use special Low Rolling Resistance (LRR) tires. Are the brake calipers stuck or dragging? This is a common problem on some hybrids because the drive system uses regenerative braking (using the electric motor as a generator so it becomes an alternator to charge the battery) and thus the brakes are used only lightly. Is the engine properly tuned? Anything that causes the engine to run longer or less efficiently can affect fuel mileage.

"More complex issues with fuel economy involve degradation of the HV battery system, which will be very slow and incremental over a long period of time," suggests Dan Cox of Midtronics, a manufacturer of hybrid battery testers. "In addition, changes in weather will be more immediate and cold weather (under 40 degrees F), in particular causes the ICE to run more than it will in higher temperatures thereby reducing fuel mileage."

But once again, these complex issues are the exception rather than the norm. In general, if there is either a rapid and/or sustained loss of mileage performance without the presence of fault codes or the dashboard warning light being on, look for the simple stuff first.

Hybrid Battery Pack Failure

So much of any automotive battery's service life, no matter if it is a 12-volt or HV battery, depends on how the vehicle is driven, what type of road conditions it sees and the overall age of the battery. That is why it is impossible to give a single answer about when a battery will fail.

Any battery's performance will degrade over time, and when this happens to an HV battery, vehicle performance is reduced. Hybrid vehicles that rely on HV batteries for power are equipped with internal systems designed to alert the operator when and only if there is a service issue.

Like all batteries, HV batteries fail because their active material and internal structures have reached a point where they have lost sufficient capacity to be considered un-serviceable.

"HV battery replacement issues are still emerging, as well as the testing criterion to accurately determine a more detailed pass/fail criterion," Cox notes.

So why are so many first generation Prius and Honda owners specifically confronting HV battery replacement? According to the experts, Toyota's HV batteries are failing because of electrolyte leakage and the resulting corrosion issues, and the Hondas are failing because of internal shorting of the cells.

The problem is there isn't much in the line of preventive maintenance that can be done. With the exception of the Gen1 Prius, which can benefit from cleaning of the corrosion and replacing bus bars on the battery pack, only careful monitoring of the cell module internal resistance and voltage differential between high and low cells while under a load can determine when a HV battery is in decline.

Using a hybrid diagnostic tool, a routine drive test will show the relationship between a known good battery and the real-time performance. If the battery test results are low or suspect, then a service decision can be made.

Other Failures

In addition to the four main service issues, other less common failures can occur. Many hybrid classes focuses on this, including classes that look at:

  • First generation Prius control units failing when corrosion from electrolyte leakage wicks down the sensor wires into the battery control module.
  • 2006-07 Toyota Highlander HEV and Lexus RX400h inverters failing and no one is sure why. Replacement cost is more than $8,000.
  • 2005 Ford Escape HEV stalling due to washer solvent in the PCM.
  • Older Honda Insights having poor grounds under the hood after they age.
  • GM Malibu BAS hybrids with leaking cells in the 36 volt NiMH battery pack.

In the end, the biggest obstacle is not the purchase of special tools. It is knowing and understanding the system you are trying to diagnose and repair.

By gaining additional skills to properly diagnose and service hybrids, and accessing subscription-based information from Honda ( and Toyota (, all of the information you need to understand and diagnose hybrids will be at your fingertips.

Jim Marotta is a freelance writer with more than 17 years' experience in the automotive industry. He currently works as a technical writer for

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