Lean trouble code P0171 & P0174 are frequently pulp up OBDII codes. The codes clearly point out it is the fuel supply system has problem. Why some people say change O2 sensor first? The code has nothng to do with sensing system. If O2 sensor has problems, ut has its own trouble codes. It should indicate O2 sensor has no problem. Could anyone please clear up this confusion. I’ve seen a lot of cases O2 sensors were changed, but lean trouble codes persistant. Is there any case or how often replace O2 sensor can solve lean system problems? Could anyone stands up and say “changing O2 sensor is a wrong treatment to lean system problems”? Where i can get an authoritive answer?
Thanks a lot.
The lean mixture codes can be caused by intake air leaks in the intake manifold after the airflow sensor or in the exhaust system before the O2 sensor.
They are saying that, because the O2 sensor is the one that is sensing the lean condition. The guess is that a sensor going bad may be giving false lean conditions. I agree with cougar, tho. Testing for a vacuum leak or air intake leak first before just blindly changing expensive sensors is a better course of action.
The problem with ‘lean condition’ trouble codes is that the computer only sees the lack of voltage from the O2 sensor. It does not know if the sensor is bad (contaminated, defective, or lead open) or if it cannot enrich the mixture enough to consume all the oxygen in the compustion mixture. Excess air in the intake can do it. Also, a faulty, plugged, or restricted fuel injector can also cause it. Low fuel line pressure can also cause this. The computer is telling you that it has used all its adaptive measures and cannot get the O2 sensor to swing to a positive voltage and so has set a code that can steer a mechanic toward the problem. Usually, mechanics start the diagnosis with installing a new O2 sensor because it is simple; does not take much time; and only costs you money. His figuring is that if it is a defective O2 sensor the problem will be fixed at reduced diagnostic cost while zeroing in on the exact problem and doing an operational check on the O2 sensor would raise your diagnostic costs significantly. I have seen a recommendation for replacing the O2 sensor(s) as a first step for any emissions related fuel system problem and then continuing from there with a known accurate exhaust monitor.
If you want authoritative, the car maker should be enough. The car maker publishes its instructions in a Service Manual. The Service Manual does NOT instruct to the mechanic to, “Change parts until you get lucky!” No, they have developed troubleshooting charts to follow, in a rational manner to find the fault.
Thanks to all of your input. I appreciate it. When the 1st time “check engine” light went on, I went to the dealer and hoped exactly what “hellokit” commented. At that time I did not do any research and knew nothing about it. I relied on the dealer to provide quality services. The result was exactly what “Researcher” pointed out. After an expensive bill, the lgihts came back with exactly the same 2 lean codes less than 2 wks after they replaced O2 sensor. BTW, this is a Lexus ES300. I’m hugely disappointed with how much they charged without systematic diagnostcs. They did very fast. That was the shortest waiting time I ever spent at the dealer and that was the 1st time my car had problem since 2001. All previous times I went to the dealer were just for services and had to wait much longer.
Apparently what they did had not fixed the problems. Do you guys think I can get my money back? I can’t trust them anymore. Thank you for your time.
I don’t think you will get your money back but you should be able to go back there and tell them what has happened. You paid to have this problem fixed but the repair didn’t work for very long. If they are reasonable they should look at the problem again and give you a break on the further repairs or do it for free. I suspect the real problem is due to an air leak somewhere like I stated earlier. It also could be dirty injectors or low fuel pressure as Researcher stated. Go back and talk with them about it.