ovoxolexus

help? my 98 ES has this emissions code (P0171) meaning she is running too lean & can't pass smog. The smog place that failed my ES did a diagnostic & smoked the engine for vacuum leaks. In result they replaced the valve cover gasket. Saying this was the cause and would pass if fixed. Still no pass even after i paid for all the repairs & tests. NOT COOL in my opinion. So i put in some new things, a new air filter, new gas filter, new PVC valve. New gas cap. New plugs, oil & filter with master tune up. Fuel pump tested & good. MAF swapped but still same issue. 2 mechanics say it's the PCU but i have doubts...I tried changing the PCU from a 98 Camry but when starting the car only for it to crank but no start. So i put the original back on. I have heard that a key reprogram has to be done in order to sync the computer to my ES. Any thoughts, comments, opinions? Thanks guys.

moonphase

According to the Haynes manual, P0171 system too lean, and the causes could be any of the following:
loose air intake hose or duct
incorrect fuel line pressure
obstruction in injector
heated oxygen sensor malfunction
MAP or MAF sensor malfunction
ECT sensor malfunction

KELVIN84

im in the same Boat and just had the VT Sensor Changed and Oil Valve control and spark plugs changed

artbuc

Close to 100% your MAF sensor needs cleaning or replacing.

tomf

In my limited experience an P0171 is an O2 sensor failure. I have two 99 ES300's. Both showed P0171. In both, replacing the O2 sensor fixed the problem. Most people say the P0171 is a MAF problem. Not for me. It was the Bank 1 sensor 1 [Air and Fuel Ratio Sensor (O2 sensor)]. O2 sensor failure is a common problem.

Additionally it was the same for my 95 Camry V6 1mz-fe engine. That's the same engine the ES300s use.

artbuc

I had (have) two cars with this engine - 99 RX300 and 99 Avy. Also take care of ES300 and 01 Avalon for friends. Over the years I have had to replace a half dozen A/F sensors (I assume that is what you meant by O2 sensor?). In every case I got specific A/F sensor codes. I have also had P0171 codes 3 or 4 times. In every case I got rid of P0171 by cleaning the dirty MAF sensor. If you search the web you will see that is a very common solution to P0171 on this engine.

tomf

I guess since he swapped the MAF I just assumed it wasn't the MAF. Unless he replaced the original dirty one with a dirty one.

A A/F sensor is an O2 sensor with higher sensitivity. From the Denso website "Oxygen sensors have a smaller range of limits in detecting and adjusting the difference from an optimum air/fuel ratio."


See this explanation from Denso http://www.densoaftermarket.com/oxygen-sensor.php

artbuc

oops, missed the MAF swap!

ovoxolexus

Thanks guys!!! I was running out of things to replace/swap emissions related. I will definitely have another look at the MAF sensor & also changing out the bank 1 O2 sensor.

StuckInVA

I honestly don't know why you would have started replacing all of those parts in the first place when you have the added diagnostic information of having a v6 with two oxygen sensors. You can obviously rule out a loose air filter hose, fuel filter, fuel pump, oil&filter, and the MAF simply by realizing that you are getting a lean signal from only ONE bank. (don't understand how the VC gaskets would cause a bank lean signal, I'm thinking the shop you went to isn't very well thought of in your community)

If I were to delve into this unfortunate episode of swaptronics with you I'd shoot for O2 (A/F) sensor first and then look at all of my bank one fuel injectors after that.

If you feel like actually checking to see if either of these are actually the problem you could always try SWAPPING them from one side to the other to see if the code swaps out for the other. Make sure to clear the code in between or you could potentially see both codes at the same time.

If you plan on doing this first make sure the fuel injectors and O2 sensors are interchangeable from either side (they should be, but I have no way of being certain as I have a 2ES) and start with the injectors to save yourself from buying the O2 gaskets if the injectors are the problem.

(sorry for the long post)

GS4_Fiend

iTrader: (22)

either you got intake gasket leaking or a defective oxygen sensor. need to get it smoked to confirm.

artbuc

Sorry, but this is totally untrue. From my personal experience and many, many others who have posted on various Toyota/Lexus car forums, it is the norm to get just a single bank P0171 lean code caused by a dirty MAF sensor.

GS4_Fiend

iTrader: (22)

its best if you put up some fuel trim numbers. that way we can access it better

moonphase

If the battery is disconnected, or the codes are erased all of the monitors are cleared which makes for even less chance to pass the smog test until they all reset. A couple of drive cycles needs to be performed so the PCM can test the different systems and set the monitors again. If one monitor won't set, the remaining monitors will not even run as they go in order as per the executive director in the PCM. Clear as mud right.

rlx101

funny, its almost like saying wear of break pads is a known problem. one wears slower than the other, but fundamentally both are wear and tear items, like almost everything else on a car. O2 sensors are rated for 100-150kmiles depending on the car, i would call it a failure only if sensors needed replacement below that mileage - otherwise its a service item.