The words in the reports, in plain English.

The small battery that powers the car's computers, lights and locks. In a hybrid or EV it also closes the relays that switch the high-voltage system on, so a weak 12-volt battery can stop a car with a full main battery from starting at all.

A small pressure tank in the brake system of many Toyota and Lexus hybrids. A pump keeps it charged with brake fluid under pressure so the electronically controlled brakes respond at once. When it leaks down, the pump runs more and codes like C1391 or C1256 can appear.

The hydraulic unit that decides how much braking comes from the friction brakes and how much from regeneration, and applies the brakes at each wheel. On many hybrids it is one of the costlier brake repairs.

A metal strip that joins the modules of a high-voltage battery in series. Corrosion on busbars and their nuts adds resistance, which heats the joint and can make healthy modules look weak to the battery computer.

A vacuum-insulated tank on the second-generation Prius that keeps hot coolant from the last drive and pumps it into the engine before a cold start, so it warms up and runs clean sooner. A valve routes the coolant, and its faults set codes like P1121.

The part that steps the high voltage down to about 14 volts to run the accessories and charge the 12-volt battery. It does the job an alternator does in a gas car. In many hybrids it sits inside the inverter assembly.

The electric motor that moves the car, which also works as a generator when the car slows down. Toyota hybrids have two inside the transaxle: one mostly starts the engine and generates, the other mostly drives the wheels.

A five-character code a control module stores when it sees something out of range, like P0A80. The letter says where (P powertrain, C chassis, B body, U network). A code is where a diagnosis starts, not where it ends.

A run of model years built on one design, like the second-generation Prius from 2004 to 2009. Parts and failure patterns tend to follow the generation, which is why the reports name model years.

The large battery that stores energy for the drive motor, at a few hundred volts. Older hybrids use nickel-metal hydride, most newer hybrids and EVs use lithium-ion. It is built from modules, and a pack is often judged by its weakest one.

A car with a gas engine and an electric drive motor that share the work, charging its own battery as it drives. It is never plugged in.

The box that turns the battery's direct current into the alternating current the motors use, and back again when they regenerate. It runs hot, so it has its own coolant loop and electric pump, and cooling faults are among its most common problems.

The high-voltage system is meant to be fully insulated from the car's metal body, and the car checks this constantly. A path to the body, from coolant, damaged insulation or a failing part, sets a fault like P0AA6 and the car may refuse to start.

A hybrid with a larger battery that is also charged from the wall, so it can drive some distance on electricity alone before the engine is needed.

A hybrid's version of a running engine. When the Ready light is on, the high-voltage system is live and the car can drive, even if the gas engine is silent. A car that will not go into Ready is holding back because something it checks at startup is wrong, from a weak 12-volt battery to a stored fault.

Cycling a nickel-metal hydride pack's modules through deep, controlled charges and discharges to bring them back into balance. It can give a sound pack more life. It cannot fix a module that has failed.

When a hybrid or EV slows, the drive motor works as a generator and puts energy back in the battery. The friction brakes do less of the work, so pads last a long time, while rotors and calipers that sit unused can rust and stick.

A shop's record of one visit: the car, the complaint, the work done and the parts. Every figure on this site is a count of repair orders from the shops' own records, each one counted once.

The shop management software the three shops keep their repair orders in. The counts on this site come from it, read every night.

How much energy a battery can hold compared with when it was new. An EV that loses range year after year is losing state of health. The Nissan Leaf shows it as capacity bars on the dash.

The unit between the engine and the wheels that, in a Toyota hybrid, holds the two motor-generators and the gears that blend engine and electric power. It has no belts or clutches like a conventional automatic.

Pumps that move coolant. Many hybrids have an electric one just for the inverter, and when it slows down the inverter overheats and codes like P0A93 follow.