Can a Lexus 400h run just on the Engine if the Hybrid System malfunctions?

markinho1975 · November 14, 2021

Hi Everyone,

Newbie to the Lexus world so apologies in advance if this sounds like a stupid question! I'm looking to buy an RX400h 2006 or 2007 and I'd like to know if in case of a malfunction the car will still be able to run of the petrol engine until the hybrid system is fixed. I've searched the internet and some sources say no it can't and others say yes it can. I'm assuming it can as this would be a very big design flaw if it couldn't as it would be inconceivable not to be able to use the petrol engine in this scenario.

I'd like to get a definitive answer on this before I go ahead and buy the car so any help here would be really welcome.

Thank you!

Herbie · November 14, 2021

No, it can't.

Three things are needed for the car to run - 1. 12V Battery 2. Petrol engine 3. High voltage traction Battery. These things all interact with and need each other.

The startup sequence is:

The 12V Battery boots the computers and gets the hybrid system into READY mode, which is basically the equivalent of a conventional car sat with the engine idling and alternator spinning.
Hybrids don't have a conventional starter motor to crank the engine. When the hybrid system wants to run the petrol engine it energises Motor/Generator 1 (MG1) and uses that to spin the engine at 1,000rpm before applying fuel and a spark to fire it.
MG1 is a 650V AC motor which is controlled by the hybrid system, so you can see that if there's a fault with the hybrid system, you can't even start the petrol engine.

Hybrids also don't use a conventional alternator and anything that would normally be driven by a belt from the engine, such as aircon compressor or power steering for instance, are instead driven by electric motors and are again controlled by the hybrid system.*

The voltage of the hybrid Battery (more properly called the traction battery) varies by model but for the 4RX like mine, it's 288V. The hybrid system uses DC/DC converters and inverters to get the various voltages required for the various systems.

The 12V systems are run, and the 12V Battery is charged, by means of a DC/DC converter that decreases the 288V from the traction Battery to about 14.5V.

The aircon compressor is a 500V 3-phase AC variable frequency motor; the power steering is, if I remember correctly, powered by a 48V motor and MG1 and MG2 are both 650V 3-phase AC.

*The hybrid system decides when to run or when to turn off the petrol engine. There are no belt-driven devices because of this - imagine losing power steering in the middle of a turn, and so on.

So you see, no hybrid system means no car. Equally so, if the petrol engine goes faulty you can't just drive on Battery power; you may get a mile or two but that would be it and if the traction Battery went completely flat you'd need a Lexus dealer to deal with it - the AA/RAC/whoever don't carry a 288V source to jump start it.

EDIT: Just to say, don't be put off by this - there are many thousands upon thousands of people driving hybrids daily.

markinho1975 · November 14, 2021

Hi Herbie,

Thanks for taking the time to answer my question in such depth, I really appreciate it, learnt a LOT! I don't think I'll be put off by this as I've heard that these cars are super reliable and the chances of having a hybrid failure are quite slim.

FYI, I'm looking to buy a used 2006/2007 Rx400h with between 100-165k miles (as long as they've been serviced properly) as I've heard that they can normally get to 250k miles without too many headaches/maintenance issues.

Thanks again for the info!

Barry14UK · November 15, 2021

Check the underside where rust can be an issue. Several members have faced expensive replacement or repair of sub fames on their cars.

Ian M · November 15, 2021

There are plenty of Toyota Pruis taxis out there with galactic mileages, so I think we can all be reassured that Toyota hybrids are pretty robust.

Trefor · November 15, 2021

I have a 2007, 105,000 miles, bought in July 2021. It is a great car and I have had no issues with it. Only thing of note is the economy has dropped a fair bit lately, I suspect a combination of E10 fuel and colder weather not helping the batteries (I have read this, but since the batteries are in the cabin area how much less efficiency can they have once the car is warmed up??). I've dropped from 33 to 34 mpg to 30-31 lately.

I would check the MOT history of any potential purchases, several I looked at had advisories on rust underneath. Mine lived in South London all it's life and is fine.

royoftherovers · November 15, 2021

7 minutes ago, Ian M said:

There are plenty of Toyota Pruis taxis out there with galactic mileages, so I think we can all be reassured that Toyota hybrids are pretty robust.

There are no Prius cars with Galactic mileages. Lunar yes, but Galactic, no.

markinho1975 · November 15, 2021

1 hour ago, Trefor said:

I have a 2007, 105,000 miles, bought in July 2021. It is a great car and I have had no issues with it. Only thing of note is the economy has dropped a fair bit lately, I suspect a combination of E10 fuel and colder weather not helping the batteries (I have read this, but since the batteries are in the cabin area how much less efficiency can they have once the car is warmed up??). I've dropped from 33 to 34 mpg to 30-31 lately.

I would check the MOT history of any potential purchases, several I looked at had advisories on rust underneath. Mine lived in South London all it's life and is fine.

Thanks for the tip, I'll be extra vigilant on the chassis when checking for rust.

markinho1975 · November 15, 2021

11 hours ago, Barry14UK said:

Check the underside where rust can be an issue. Several members have faced expensive replacement or repair of sub fames on their cars.

Thanks I'll make sure to check that properly.

serbarry · November 15, 2021

I do not have a Lexus but I have a 2004 Prius with 164,000 miles which has more or less the same hybrid system as the Lexus. In nearly 12 years that I have been driving it the only problem I have had was with the power steering module that needed to be replaced. It has always been serviced by an independent mechanic and only once by the Toyota dealer.

royoftherovers · November 15, 2021

19 minutes ago, serbarry said:

I do not have a Lexus but I have a 2004 Prius with 164,000 miles which has more or less the same hybrid system as the Lexus. In nearly 12 years that I have been driving it the only problem I have had was with the power steering module that needed to be replaced. It has always been serviced by an independent mechanic and only once by the Toyota dealer.

Great Post Sergio.

Will that be a Mark 1 that you have ?

serbarry · November 15, 2021

17 minutes ago, royoftherovers said:

Great Post Sergio.

Will that be a Mark 1 that you have ?

No it is the second generation Prius 2004-09.

Steven Lockey · November 15, 2021

1 hour ago, markinho1975 said:

Thanks I'll make sure to check that properly.

Yep, had to replace my entire exhaust from the cats back on my GS450h.

Also shocks tend to need replacing at some point. Battery is the other one that tends to go.
All are replaceable with non-OEM versions 🙂

Herbie · November 15, 2021

1 hour ago, Steven Lockey said:

Also shocks tend to need replacing at some point. Battery is the other one that tends to go.
All are replaceable with non-OEM versions 🙂

Just for the sake of clarity (because Mark, the OP, is new to hybrids) - Steven is probably talking about the 12V Battery, not the traction battery :thumbsup: