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I changed my spark plugs about a week ago. Done around 700 miles since and car seems to have been fine. But all of a sudden today during a long drive stuck in traffic the engine management light came on. Plugged in my diagnostic tool which has always been very reliable. I've got the following stored code and pending code:

Stored Code: P0174 System too lean bank 2

Pending Code: P0171 System too lean bank 1

From doing my research it can be number of things from MAF Sensor, PCV hose, vacuum leak in intake or o2 sensors. My MAF sensor is rather new. Intake pipes I made sure were connected properly. So question is is it likely to be the o2 sensors? My car is about 10 years old and has the original o2 sensors from factory with about 130k miles. Is there a way to test them or should I replace certain o2 sensors?

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Have you thoroughly checked all hoses are correctly tightened after doing your plugs. Air Filter box and contents all good? You could try a solvent clean of the MAF ... it won't do any harm to cancel those out of the equation.

As far as I recall Lexus points towards changing the upstream sensors around the 80,000 mls mark. 

That's not to say it is mandatory as you will plenty of people going over 100,000 with no issues. I did both my upstream sensors a little while back. Think I was on 75,000 or so and the car was much improved for it too. I did these as a precaution and had noticed more smoke (at idle) from the exhausts.

It was here if you want to have a looksee at what is involved

 

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.... also - if it is your sensors then its best to change both at the same time - doing so does make a difference to the running of the car.

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Have you got your last MOT certificate to see what the O2 readings were?
They don't tend to fail in a short space of time, the levels creep up, it's worth seeing if your readings were at the upper limits (but within tolerance) at the last MOT.

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Thanks for replies guys. I did check the hoses and everything. Everything seems ok in terms of hose connections. There are the smaller hoses related to removing the manifold that just slot in and the spring clips that clip them on seem to just easily spin so not sure if it's worth replacing them.  Lot of literature seems to point the P0174 code as a air leak more than a faulty O2 sensor. The only other thing I can think of is the gaskets of the manifold. I replaced these with brand new ones and also replaced the throttle body gasket with brand new one so not sure if there is any way to find out if any of these are the culprits?

The car doesnt seem to hesitate when picking up or anything. I cannot seem to find where I placed my previous MOT's to compare the emissions reading will try and locate them. I cleared the codes and drove about another 600 miles and they came back. 

With my OBD tool, i use dash command which allows me to monitor live data, is there anything I can look out for to determine what is likely to be the cause?

IS200 Newbie: I read your write-up when you changed your o2 sensors, you gave a denso part number which seems different to the part number lexus give. Is that Denso one same as OEM ones?

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Also noticed after clearing the code. After some driving the p0174 code comes back in the pending codes and stays pending and then eventually throws a stored code which then also seems to trigger a p0171.

The p0174 seems to refer to bank 2. What exactly is bank 2?

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The v6 configuration has 2 'banks' of 3 cylinders that slant left and right. Standing at the front of the car Bank 1 is to the left and Bank 2 is to the right.

Go back to the link I gave you and you will see some diagrams that should make it clearer to grasp.

 

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here ya go

586f6dfa40ba0_Sensorlocations-page-001.jpg.acb25de718ddb5941b97381b5a52e7b4.jpg

586f6e619d166_o2sensoridentification-page-005.jpg.823979f3d6b18b69e07a738f288bc005.jpg

Look at the left hand diagram above. We have the 4GR-FSE

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Assuming you have 2007 model . These are the indicated sensors for your car.

Bear in mind there are Upstream and Downstream sensors. The Up and Down are quite different animals.

Here is screen shot from Denso website for 2007 Lexus IS 250

screenshot.thumb.jpg.1688b2d24c8d92e5118d938ddee74534.jpg

 

Newbies part numbers look correct. It may be Lexus quoted you for Downstream sensors by mistake?

Here's the screenshot from Rockauto parts in the USA, again for 2007 IS250

rockauto.thumb.png.3c0f7a7668b31b735490b6011a017cc0.png

I've highlighted your upstream sensors

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Awesome Normski, that's great pieces of information. seeing as my car is way over 100k and still on original upstream sensors, i'm guessing replacing them is a good thing anyway even if its not the cause of this error code? is it also worth replacing the pcv valve as it is a very cheap part anyway? where exactly is the pcv valve located?

in regards to the error, seeing as system too lean on bank 2 error keeps coming up, if it was a air leak, should i be looking around the bank 2 area if the v? there isnt much there other than the inlet gaskets and a hose right behind the inlet manifold.

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the rockauto website is really useful. I found the denso part number: 234-9051 as Newbie gave, and if you click on info about alternative OE numbers, in the link listed right at the bottom: http://www.rockauto.com/en/moreinfo.php?pk=1606992&cc=1432808&jsn=384 the OEM number that lexus gave me is listed there so it seems this is the correct DENSO part. 

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The PCV valve is worth changing at the mileage. It's peanuts ... so why not? That is also associated with a smokey exhaust. If you do Bank 2 Sensor 1, then with what you have removed its a doddle to do. To do on its own its a bit of a PITA as its awkward to get at ... unless you have some crow foot spanners.

I'm no expert but it sounds like your bank 2 sensor wants doing. That is the more awkward of the two upstream sensors. Bank 2 requires you to remove the battery, its tray and the electrical parts underneath it.If it were me I'd do both Upstreams. Bank 1 is Peasy by comparison.

Take a read of the whole of that link I gave you .Both Newbie and I have put up pretty good work arounds and pictures. You should get all you need from that. You'll need that sensor socket also. I've shown pics and method for the PCV too.

Just be wary of the add ons from buying through Ebay as it can jack the price up quite a bit.A lot of Ebayers will tell you there is $35 or more import duty to pay on top of postage.

When I bought mine from Rockauto there were no extras at all...... maybe i was lucky.

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**** .... this is worth looking at

http://www.ebay.co.uk/itm/2PCS-O2-Air-Fuel-Ratio-Sensor-For-2006-2009-Lexus-IS250-IS350-GS300-Denso-Toyota-/382205995519?hash=item58fd40a9ff:g:vs0AAOSw1blZnfx~

Message them to be sure they are correct fit. Even if they take a month to arrive it's darned good value.

I've just made 3 Ebay purchases from China. One item was 10 days the other two were just under 3 weeks. Bought a fair bit from China not had one problem thus far. Great for LED bulbs.

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I messaged the US one because noticed the box had chinese writing to find out if they are genuine denso or chinese replcias and they replied saying they are replicas from china LOL. I think it's better to stick to genune DENSO dont you think?

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I understand your reservations. You know the Denso ones will be correct.

China makes a lot of very good kit. Spencer Kelly's 'Click' programme on BBC1 has been an eye opener from time to time.

The Denso type are sourced from Japan. They supply loads of kit to Toyota. Wouldn't it be ironic if they outsourced some bits to China? :whistling1:

Are you planning on doing them yourself?

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4 minutes ago, normski2 said:

The Denso type are sourced from Japan. They supply loads of kit to Toyota. Wouldn't it be ironic if they outsourced some bits to China? :whistling1:

LOL, I would not be surprised. I did buy a few sets of different brand chinese D4S bulbs and kept having issues with every set. Works for a good few months around 6 months or so then starts to flicker, eventually settled for Genuine Philips and they have so far lasted me 2 years without issues. So hence why I am a bit reluctant to by chinese replicas. Might work nicely for a while but don't want to have to replace it again very soon

Are you planning on doing them yourself?

Yeap planning to do it myself.

 

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Good man!

Make sure you have

  • Read Newbie's (shaun) guide first. In fact, print off some pics or view them on an Ipad or similar. They are extremely useful.
  • The 22mm oxygen sensor socket ...you can get 1 from Ebay for £4 or so. Just seen this type ... rather than the offset type. May be a good bet and it will do the Downstream sensor too if the need ever arises. The downstream has a protective cowling around it.

       NEW 22mm 3 8'' Drive CRV Oxygen 02 Sensor Socket Removal Lambda Lamda Tool Kit

  • A strong socket set. You'll need an extension to reach about 600mm above Bank 2 sensor to get above the engine and surrounding bodywork/components. 
  • A length of pipe that fits over the socket handle, to give more leverage.
  • Some Copper/Anti seize grease for the new sensors at the top seat of the thread. There should be some already on the sensor.
  • Sprayed the old sensor threads with Plusgas/WD40 or similar for several days before you want to start removal. They can be little so and so's to undo.
  • Method of taking pictures, more on bank 2 side, it helps with re-assembly.
  • For the PCV you'll need a 19mm spanner and a dab of loctite for the threads. Just out of interest, compare the rattle of the new and old one.
  • Oh, and a bradawl or tiny electricians screwdriver for the bl***y awkward connectors under the battery tray.
  • A lot of patience .... and stay calm at all times.

      Good luck and let us know how it goes.

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Thanks Normski, Great tips.

I read somewhere that you can measure the resistance in ohms of the upstream sensors and apparently the ohm readings should be between certain amounts and if it's not it indicates the sensors are no good. Is there a way to get to the connectors of the upstream sensors easily without removing too much so that I can measure the resistance?

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rockauto has 2 brands of the PCV valve, which brand did you end up getting? is it any good compared to genuine OEM?

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This was my PCV. Standard Motor Products from memory. About £5 or so. 

IMG_0616.JPG

 

You could try attaching crocodile clips to the wires at the top of the sensor. 

I wouldn't let that stop me from changing them though. You'll be pretty hacked if you get the VSC and limp mode when they give up at a later date. If i recall correctly they are scheduled for changing at around 80,000 mls.

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15 minutes ago, normski2 said:

You could try attaching crocodile clips to the wires at the top of the sensor. 

I wouldn't let that stop me from changing them though. You'll be pretty hacked if you get the VSC and limp mode when they give up at a later date. If i recall correctly they are scheduled for changing at around 80,000 mls.

Yeah I think I will definitely replace the upstreams and the PCV just because they have done high mileage anyway. But I thought it would be nice to measure the resistance of the old ones and see if they are within the thresholds to see if they were actually faulty.

That's the PCV valve I saw on rockauto too. How many miles have u done with the new PCV? does it need a new washer or anything?

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About 4000 mls

Washer is already on it

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26 minutes ago, normski2 said:

About 4000 mls

Washer is already on it

Awesome thanks. I think I will just place my order with RockAuto seems to be the cheapest option. 

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Got to agree with Normski - its a well worthwhile job to do - you should see a difference in the running of the car after replacing both. I never done the PCV valve when changing the sensors on mine - to be honest I did not know it could be done. Job for the future.

The hardest thing about replacing bank 1 is taking the engine covers off.

Bank 2 is a bit more tricky, not because theres mountains of kit to remove, because theres not a lot of space to work in - you do need to remove battery and the electronic kit underneath which I think is the ECU so don't bang it around - take your time and you will see how easy it is to do. The sensors I bought came with a small sachet of thread copperease grease type of stuff so there may not be a need to purchase any specific for the job (not that its expensive stuff anyway)

Remember to disconnect your battery before unplugging connectors - but as you progress you will need to anyway

 

Let us know how you get on.

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Also,

This sensor, Denso Air/Fuel Sensor 234-9051, will fit both Bank 1 and 2. The differences between the banks if you are quoted two part numbers is probably due to the length of the sensor wire but this part number has a wire long enough to fit in either bank.

You will see when you take them off the car that one sensor wire is a bit shorter than the other.

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