2000 Protege rough idle P0171 only in Drive
 

Runs fine going down the road P0171 code. I replaced the O2 sensor 1. Same thing, rough idle. Only does this in Drive. Go to Neutral or Reverse and it smooths out. Noticed that O2 sensor 1 voltage goes to 0 & STFT goes to O, only in Drive. No vacuum leaks, air hose is good. Need to know what the 0 readings mean only in Drive HELP!! PLEASE!!!


Idle @2K rpm

MAF .22 .55-.61 @ 2K rpm

STFT -.7 to -.10 -.17 to -.21

LTFT 25.0 25.0

O2 sen1 .06-.6.6 .11-.7

O2 sen2 .5 .05

mine is doing the same exact thing rough idleing in D with 0 volts at 02 sensors at D idle too with check engine light on for p0171. Its not the 02 sensors, intake hose, MAF sensor or a vaccumm leak, am at a lost too. I have the same 2000 protege like yours too. I heard its a common problem on these protege.

If i were you check your MAF sensor and the air filter box make sure the air filter box is sealed right.

Principally there are 2 ways to arrive at a lean condition: too much air and too little fuel.

You guys seem to have covered the first way.

Well, Can't see where it could be the fuel filter, as it runs fine going down the road. No leak in the air box, clean filter. Just O2 sensor 1 goes to 0v in drive and STFT reads 0. When you take it out of drive, the readings return to normal and the engine smooths out in N,R,or P. Beginning to think that it may be the gear selector switch(or neutral safety switch). By the wiring diagram, the Drive signal goes directly to the PCM. You think this is possible?

did you check your MAF sensor thats a common problem for these cars, 90% of the time p0171 is usually for a faulty or dirty MAF sensor on this car. In my case is not because i replaced it and still doing the same thing but it may make a difference with you.

Yea I check it and cleaned it. No change. Still gets a signal from it. Around .22 at idle and increases to. .51 -.61 @2k rpm even in Drive. Just out of curiosity, I unplugged it and the engine almost dies. It had an EGR code for insufficient flow. I cleaned it out and check it was closing and cleaned the vac port that goes to the sensor, that code went away. So no leaked there. Still thinking gear selector switch. I'll find out around noon when the switch comes in.

Yes, I think this is possible. If the gear selector switch gives a faulty signal the PCM can't figure out the correct amount of fuel. With computers it is "junk in=junk out".

Hey well found my problem the brake booster hose was bubbling and cracking up and caused a vacuum leak. Replaced with a like new condition junkyard brake booster hose, check engine light off no more rough idle.

Check your brake booster hose.

Listen to mazpro00. It makes a lot of sense. Most of the time you are stepping on the brake when you put it in D. Then the idle speed goes up which creates more vacuum.....
Often the rubber hoses get hard and cause vacuum leaks where they are attached to nipples and connectors. They have to be flexible to form a seal.