P0171

[Yisroel K]

Hi guys I believe my downstream o2 sensor is not working and I just got a p0171 code on my 1999 4runner sr5. The idle is a little rough and the gas mileage went down alot.
Should I replace both sensors? How to know if my car has a af sensor or o2 sensor?
I cleaned my maf.

 

[spartacus]

Hi guys I believe my downstream o2 sensor is not working and I just got a p0171 code on my 1999 4runner sr5. The idle is a little rough and the gas mileage went down alot.
Should I replace both sensors? How to know if my car has a af sensor or o2 sensor?
I cleaned my maf.

Sensor one bank one is the af sensor before the cat, sensor 2 the 02 sensor after the cat.
Clean maf sensor, Make sure you eliminate vacuum leaks and other causes of p0171.

 

[jgue467]

Hi guys I believe my downstream o2 sensor is not working and I just got a p0171 code on my 1999 4runner sr5. The idle is a little rough and the gas mileage went down alot.
Should I replace both sensors? How to know if my car has a af sensor or o2 sensor?
I cleaned my maf.

As Spartacus said, first take care of the intake air leak (cause of 0171) Then consider the sensors. Sensors can be tested against FSM so one isn't just guessing they're bad.

Take a really close look at the rubber intake component closest to the metal intake runners. It receives tremendous heat and cracks over time, often not visible but it is cracked letting in un-metered (since it is after maf) air messing up the a/f mixture causing the lean condition.

If you replace that part, go with a toyota oem, its only about $25 but fits perfectly.

 

[Yisroel K]

Sensor one bank one is the af sensor before the cat, sensor 2 the 02 sensor after the cat.
Clean maf sensor, Make sure you eliminate vacuum leaks and other causes of p0171.

As Spartacus said, first take care of the intake air leak (cause of 0171) Then consider the sensors. Sensors can be tested against FSM so one isn't just guessing they're bad.

Take a really close look at the rubber intake component closest to the metal intake runners. It receives tremendous heat and cracks over time, often not visible but it is cracked letting in un-metered (since it is after maf) air messing up the a/f mixture causing the lean condition.

If you replace that part, go with a toyota oem, its only about $25 but fits perfectly.

I checked for vacuum leaks with a Halloween smoke machine and didn't see or hear any leaks.
My downstream o2 sensor after my cat is not reading any volts on a obd2
I tested my upstream af sensor and it had ohm reading of 15 v. I forgot to test the o2 sensor.

 

[LittleCaesar]

If you have the ability to read live data with a code reader, see what the difference is between your short and long term fuel trims once warmed up. But if you aren’t getting any voltage readings for a particular O2 sensor in different driving scenarios, I would start there.

 

[Yisroel K]

If you have the ability to read live data with a code reader, see what the difference is between your short and long term fuel trims once warmed up. But if you aren’t getting any voltage readings for a particular O2 sensor in different driving scenarios, I would start there.

My short trim was a steady 19.5%
Long trim was a steady 39.8%
Maf about 4 g/s when accelerated it went up so don't think maf is bad.
O2s1b1 was reading 0.645v
O2s1b2 was reading 0.000 - 0.005v

 

[LittleCaesar]

My short trim was a steady 19.5%
Long trim was a steady 39.8%
Maf about 4 g/s when accelerated it went up so don't think maf is bad.
O2s1b1 was reading 0.645v
O2s1b2 was reading 0.000 - 0.005v

The trim number difference between the long and short indicates there is a real problem, and without any voltage to the rear, I would start there. Crawl under to make sure the wires are still good (haven’t been cut, etc.). If the sensors are original, I’d replace them.

 

[Yisroel K]

The trim number difference between the long and short indicates there is a real problem, and without any voltage to the rear, I would start there. Crawl under to make sure the wires are still good (haven’t been cut, etc.). If the sensors are original, I’d replace them.

Which sensors are the correct one I looked up a few on Amazon and it said it does not match the car. I have one cat so I'm assuming not Cali rated.
Do I need a a/f sensor and o2 sensor or 2 o2 sensors?

 

[LittleCaesar]

Which sensors are the correct one I looked up a few on Amazon and it said it does not match the car. I have one cat so I'm assuming not Cali rated.
Do I need a a/f sensor and o2 sensor or 2 o2 sensors?

Find My Part

 

[kball]

2001 & 2002 4WD & 2WD V6; Front: 234-9002, Rear: 234-4261



2000 Cali 4WD manual V6; Front: 234-9001, Rear: 234-4153 2000 Cali 2WD manual V6; Front: 234-9001, Rear: 234-4155

1999 & 2000 Cali auto, 2WD/4WD V6; Front: 234-9002
1999 & 2000 Cali auto, 2WD V6; Rear: 234-4155 1999 & 2000 Cali auto, 4WD V6; Rear: 234-4153


1999 California manual V6 2WD/4WD 4Runner; Front = 234-9003
1999 California manual V6 4Runner; 2WD Rear = 234-4155 1999 California manual V6 4Runner; 4WD Rear = 234-4153

1999 & 2000 49 State 2WD & 4WD auto V6; Front = 234-4162 1999 & 2000 49 State 2WD & 4WD auto V6; Rear = 234-4162

1999 & 2000 49 State manual V6 2WD; Front: 234-4162 1999 & 2000 49 State manual V6 4WD; Front: 234-4161 1999 & 2000 49 State manual V6 2WD & 4WD; Rear: 234-4154


1996-1998 4WD manual V6-3.4; Front = 234-4161
1996-1998 4WD manual; Rear = 234-4154

1998 2WD ALL Front = 234-4162; Rear 234-4154

1996-1998 Auto 4WD V6 3.4; Front = 234-4162
1996-1998 Auto 4WD V6 3.4; Rear = 234-4154

1997 Auto 2WD; V6 3.4 Front = 234-4162, Rear = 234-4153 1997 Manual 2WD; V6 3.4 Front = 234-4161, Rear = 234-4153


1996 All 2WD; V6 3.4 Front = 234-4162
1996 All 2WD; V6-3.4 Rear = 234-4153

2.7 & 3.4 V6 4Runners:
2000 California manual L4-2.7; Front = 234-9001
2000 California manual L4-2.7; Rear = 234-4622

1999 & 2000 California auto 2WD/4WD L4-2.7; Front = 234-9002
1999 & 2000 California auto 2WD L4-2.7; Rear = 234-4622 1999 & 2000 California auto 4WD L4-2.7; Rear = 234-4154


1999 California manual L4-2.7; Front = 234-9003
1999 California manual L4-2.7; Rear = 234-4622

1999 & 2000 49 State 2WD/4WD L4-2.7; Front = 234-4162
1999 & 2000 49 State 4WD L4-2.7; Rear = 234-4154 1999 & 2000 49 State 2WD L4-2.7; Rear = 234-4162

1998 2WD L4-2.7; Both Front & Rear: 234-4162

1996-1997 All 4WD & 2WD L4-2.7; Front = 234-4162
1996-1997 All 4WD L4-2.7; Rear = 234-4154 1996-1997 All 2WD L4-2.7; Rear = 234-4153