High-voltage battery, cooling, inverter and brakes: the common problems of hybrids, and what to look at in a used car inspection before you buy.
Hybrid cars have fairly earned a reputation for reliability, and many easily pass 300,000 km. That doesn’t mean they have no weak points. A used hybrid has every component of a conventional car, plus a second drive system with a high-voltage battery, electric motors and power electronics. Below we explain where problems usually appear and what to check before you buy.
Which kind of hybrid are you looking at?
The word “hybrid” covers three quite different cars:
- Mild hybrid: a small system, usually 48 volts, with a starter-generator on the engine belt. It helps with acceleration and start-stop, but the car can’t move on electricity alone.
- Full hybrid: a high-voltage battery and electric motors that can drive the car for short distances. It charges itself, from the engine and from braking.
- Plug-in hybrid (PHEV): a larger battery that also charges from the mains, with an electric range of several tens of kilometres.
The bigger the electric part, the more its condition should weigh in your buying decision.
1. The high-voltage battery
It is the most expensive component and the first thing buyers worry about. Like any battery, it gradually loses capacity with age and charging cycles. Older hybrids often use nickel-metal hydride (NiMH) batteries, while newer ones use lithium-ion.
The battery is made up of many modules connected in series. The usual problem isn’t that the whole battery “dies”, but that one or two modules weaken. The management system then sees an imbalance in voltage between the modules, limits electric operation and, at a later stage, turns on a warning light.
Signs worth watching for:
- the battery charge indicator rises and falls very quickly,
- the petrol engine starts very often, even in gentle driving,
- fuel consumption is noticeably higher than expected for the model,
- on a PHEV, the electric range is much shorter than the official figure, even on a full charge.
The real condition of the battery can’t be seen with the eye. It needs a diagnostic tool that reads fault codes and, depending on the model, the voltages and temperatures of the modules.
2. Battery cooling
A less well-known but very common problem. On many full hybrids the battery is cooled with air from the cabin, through a fan and a vent usually located near the rear seat. Over the years, the vent and the fan fill up with dust, fluff or pet hair.
With the cooling blocked, the battery heats up, the system limits its power to protect it and it ages faster. The good news is that cleaning it is a simple, cheap job, as long as it gets done. A loud, constant fan noise from the back of the cabin is a sign that something is wrong.
3. The inverter and its cooling circuit
The inverter converts the battery’s direct current into alternating current for the electric motors, and back again. It produces a lot of heat and usually has its own cooling circuit, with a separate reservoir and an electric water pump. If the pump stops or the coolant is low, the inverter overheats and the car may go into a reduced-power mode. Check the level in the second coolant reservoir and ask whether the coolant has ever been changed.
4. The 12-volt battery
Every hybrid also has a conventional 12-volt battery, which powers the electronics and “wakes up” the hybrid system. When it weakens, the car may not go into READY mode or may show odd, unrelated faults. It is one of the most common causes of a “breakdown” and one of the cheapest to fix, which is why we always test it.
5. Brakes that don’t get much work
Hybrids do much of their braking with the electric motor, which acts as a generator and charges the battery. The pads therefore last a long time, but the mechanical brakes are used little and stay exposed to moisture. The result is rusty discs, seized callipers and slide pins, and noise. Look at the surface of the discs and ask for the brake fluid change history.
6. The combustion engine
In a hybrid the petrol engine switches off and on very often and frequently doesn’t reach operating temperature, especially on short trips. That encourages moisture, oil dilution and carbon deposits. On PHEVs that are charged every day, the engine may barely run, leaving fuel sitting in the tank for months. Regular oil changes with the right specification matter even more here, and an oil analysis shows whether there is a problem.
7. Gearbox and belt
Many full hybrids use a planetary transmission (e-CVT) with no clutch, which is generally very durable. Others, though, use a dual-clutch or conventional automatic gearbox, with the known wear issues of those gearboxes. On mild hybrids the starter-generator is driven by the engine belt, so the belt and tensioner carry a higher load and need checking.
8. Charging on plug-in hybrids
On a PHEV, check the charging socket for burnt or corroded contacts, the cable that comes with the car, and that charging starts normally. The on-board charger is an expensive part, and charging that keeps cutting out should be investigated.
Take extra care with former taxis
In the Greek market there are many hybrids that have been used as taxis, with very high mileage. They aren’t necessarily a bad buy, as they have often been serviced regularly, but the battery, suspension, brakes and interior have worked hard. The price should reflect that, and checking the true mileage matters a great deal here. See how to spot odometer tampering.
Warranty and history
Manufacturers often give a separate, longer warranty on the hybrid battery, and some extend it as long as the hybrid system is checked regularly in their network. Ask for the relevant documents and check whether the warranty transfers to the new owner. A recent hybrid system health report from an authorised workshop is a valuable piece of evidence.
Safety: the orange cables carry high voltage. Don’t open or touch components of the hybrid system; any work must be done only by trained technicians.
What to do on the test drive
Start with the car cold and make sure it goes into READY normally. In town, the switch between electric drive and the petrol engine should be smooth, almost imperceptible. Notice whether the car moves on electricity at low speed, how the battery indicator behaves and whether you can hear the cooling fan. More in our article on the right test drive.
In the CarCheck Full Inspection the diagnostic check reads the fault codes of every module, including the hybrid system, followed by a test drive and an oil analysis. See what the used car inspection covers or book an inspection.
The used car inspection for a hybrid
A well-maintained hybrid is one of the safest choices on the used car market. The problems that appear are usually known and predictable, as long as you look for them before you buy and not after. For the other points worth checking, see our 10 things to check before buying a used car and our comparison of diesel or petrol.
Frequently asked questions
How long does a hybrid battery last?
On many hybrids the high-voltage battery lasts as long as the car, but it gradually loses capacity with age and mileage. Its condition depends a lot on how it was cooled and maintained, which is why it should be checked before you buy.
Is a former taxi hybrid a good buy?
It can be, but it usually has high mileage and heavy use. Ask for a full service history and pay particular attention to the battery, its cooling and the gearbox.
Does CarCheck check the hybrid system as part of the used car inspection?
Yes. In the Full Inspection we read the fault codes of every module, including the hybrid system, with a diagnostic tool, followed by a test drive and an oil analysis. The inspection is done on site, anywhere in Attica. Book online.
