RX400H engine light comes on

[400h08]

3 weeks ago, the engine light comes on (2008 RX400h).  I took it to the dealer for diagnosis.  The service advisor said one of the 2 senors are defective and needs to be replaced.  They do not have the parts, so I said to delay it until June when my normal checkup is due.  After a couple of weeks, the engine light goes off by itself, and everything seems fine now.  

I am thinking of not replacing the senor because it is quite an expensive repair/replace.  What do you owners think?

[lenore]

Did they give you the codes?  We cant help you without them.

[400h08]

No, not the actual code, and I did not know enough to ask for it.  What is said on the invoice is:

"Checked for codes B1 S2 not working, Rec to replace both B1 S2 and B1 S1"

I guess I'll have to ask them if I still need to replace it.

[user800]

B1 S2 refers to the Bank 1 Sensor 2, basically the oxygen sensor.  If the engine light is off it means whatever caused it to come on is no longer causing an issue. Keep driving, it's all OK.

[400h08]

Thanks for the info.  What is B1 S1?

So my engine light came on again after 2 months of driving since its first occurrence. I had it replaced last June. The dealer tried to make me replace B1 S1 as well and I refused.  

[user800]

The V6 engine is divided into two halves, the left side and the right side. One side is referred to as Bank 1 and the other side is Bank 2. Each bank has an exhaust pipe coming out of it, and each of these pipes has an oxygen sensor (Sensor 1, AKA S1) followed by a catalytic converter, followed by another oxygen sensor (Sensor 2, aka S2). So B1 S1 = Bank 1, Sensor 1

Which sensor did you replace already?

When you replace an oxygen sensor it is recommended to replace both sensor 1 and sensor 2 because they work together as a system. If one is old and the other is new then you can get issues. But before you go replacing the other oxygen sensor, you need to have the car scanned to get the code that triggered the engine light to come on. It may be related to the older oxygen sensor but could also be something completely different. 

So you need to get the car scanned either at the dealership (preferably) or local mechanic and get them to print out the codes (or write them down or take photos on your phone). Let me know what the code(s) is so I can tell you what to do next.

[400h08]

Thanks for your detailed explanation.  I am not doing the work myself.  Just want to understand it.  It has been 4 months since the dealer replaced B1S2 only.  I'll leave it at that until the light comes on again.  The dealer first recommended to replace B1S2 and B1S1 together, but after I hesitated, they say I can just replace B1S2.  

What happens if I did not replace the defective sensor at all.  The dealer said doing so will eventually increase my mileage.  This thing id very expensive to replace, increase mileage is cheaper option.  What do you think?

[user800]

If any of the oxygen sensors is faulty it will trigger the engine light to come on, as the engine computer relies on the oxygen sensors to determine the correct amount of fuel that needs to go into the engine to achieve ideal combustion. If the oxygen sensor is not working, the engine has no idea if there is too little or too much fuel going into the engine which leads to higher fuel consumption and also higher emissions. If too little fuel is going in, the engine will run rough, hesitate on take off, and have less power, and if there is too much fuel going into the engine, all that extra fuel ends up in the catalytic converter causing the catalytic converter to fail and it is expensive to replace the catalytic converter. I work with Jeep and I've seen them cost $5000+ just for the part, not including labor. Don't know how much they are for Lexus.

The oxygen sensor should not be that expensive. The dealer is probably charging you way more than what it actually costs - all dealerships do that, especially for genuine parts.  You can always get an aftermarket sensor or find one on eBay. Buy the sensor yourself and get the local mechanic to fit it for you - it will end up a lot cheaper.

When you get the chance, get the car scanned for codes, if there is any codes for the older oxygen sensor, save up for it over 2 or 3 months, and then replace it. But it is not good to leave it as is IF the oxygen sensor is faulty, to prevent damaging the catalytic converter. All the best!

[400h08]

Thanks for the detailed explanation.  Only the B1S2 is replaced back in June.  It is working ok.  Mileage is like normal.  So do I have to replace B1S1 now or wait till the light comes on again?

[user800]

Scan the car first and if it shows a problem with the B1S1 sensor then you should replace it as soon as you can. But if there is no issue with that sensor, then wait till the engine light comes on. When you get the car scanned, ask them to print out the codes and then clear the stored codes, that way old codes are removed from the car's memory. Makes it easier to diagnose any problems in the future so that outdated codes don't show up next time you scan the car.

Edited October 14, 2017 by user800