rhworkman

I recently purchased a used 2001 Lexus LS430 in another state. I went to have the emissions inspection and it failed with the following errors: P0171 & P0174 ("System to lean" errors for each side of the engine). Based on a thread I found here, I first attmepted to claen the Mass Air Flow sensor, and then when that didn't work, I replaced the Mass Air Flow sensor (this past weekend). The car once again failed emissions for the same errors. The attendent at the emissions test shop suggested that I needed to make sure that I had "completed the correct number of cycles" in order to reset the computer.

Questions:
1) does the computer need to be re-set after changeing the MAF sensor?
2) How do I re-set the computer?
3) Any other suggestions on passing emmisions based on above data?

Thanks in advance, RHW

archangels

yes any changes you must remove negative battery cable for about a minute to reset the ecu, then put back on the cable and put ignition on on position for 5seconds or untill it stops beeping then start engine and your ecu should now be reading new airflow and etc that the maf sensor reads as current flow after new install...

dyerjb

Thanks for the suggestion, it cleared my code but I'm still hesitating when I press gas hard.

Twix

It's not hesitating...it's learning...
Drive it for a week or so and it should become more "live" (depending on the way you drive)...

StanVanDam

The procedure for resetting the ECM (aka PCM) A/T learned values is to disconnect the battery cable for 5 minutes, then turn on the car and idle until normal operating temperature, then drive with several accelerations from a stop with light throttle application until proper transmission shifting is verified.

The drive cycle patterns for the various OBD-2 monitors are all different and require various combinations of idling, maintaining speeds, selecting specific gears, etc.

For P0171/174, the pattern is start engine, idle for 100 seconds or more, drive at 40kph for 20 seconds or more, return to 0kph and idle for 20 seconds or more, repeat 2 more times, then idle for 30 seconds, then turn the car off, then repeat all of those steps again. After the final 30 second idle, the MIL should light up if there is a problem.

Troubleshooting steps for P0171/174 are:
1) Check air induction system, repair/replace
2) Check injector injection, replace injector if faulty
3) Check MAF and coolant temp sensor, repair/replace
4) Check for spark and ignition, repair/replace
5) Check fuel pressure, pump, pressure regulator, pipe line, filter
6) Check for exhaust gas leakage, repair/replace
7) Warm up O2 sensors by idling at 2500rpm for 90 seconds, then idle normally and check waveform of O2 sensor voltages. Should be slightly under 0.4V to slightly over 0.55V
8) Check for open and short circuits in harness and connectors between ECM and O2 front left and right O2 sensors, repair/replace harness, or replace O2 sensors.
9) Clear MIL, perform confirmation driving pattern again.
10) Replace ECM
11) Did the vehicle run out of fuel in past? P0171/172/174/175 is caused by running out of fuel.

Judging from the age of the car, I would have suspected the O2 sensors first. My front right heater element died at 83500km, exactly 8 years 0 months in-service. Front right is an easy replacement. I didn't replace any other O2 sensors, no problems 9500kms and 12 months later.