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Toyota Service
Toyota Service, Toyota Expert
Category: Toyota
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2001 Toyota ghlander check engine light with code P0171

Customer Question

2001 Toyota Highlander check engine light with code P0171 (Bank 1 lean) I changed Mass Air Flow sensor did not help. Same code set changed o2 sensor bank 1 did not help. same code set. It runs fine sometimes when check light is on but at times it will hardly move.
Submitted: 1 year ago.
Category: Toyota
Expert:  Toyota Service replied 1 year ago.

Hello. Welcome to Just Answer. Please allow me to assist you. Did you install a genuine Toyota Mass Air Flow sensor? We have observed a few 'non-Toyota MAFs' that are bad out of the box.

Expert:  Toyota Service replied 1 year ago.

This is what sets the code: Sum of short FT and long FT is more than 35% when following conditions are met (2 trip detection logic):
(a) Coolant temp: 70°C (158°F) or more
(b) Engine rpm: 1500 rpm or more
(c) Vehicle speed: 100 km/h (62 mph) or less

Expert:  Toyota Service replied 1 year ago.

You may have a coolant temperature sensor (for the fuel injection) that is not within the limits of the computer's programming. IE the coolant temps should be above 32 degrees. If the coolant temp is reading 30 degrees, and the intake air temp (measured by the MAF is 150 degrees, this is theoretically impossible as this condition cannot take place. The computer will set the light as the parameters are not witrhin the designed programming. This may not set a coolant temp sensor code, as it is not shorted or open. I woul dbe looking at the actual, real time coolant temperature via a good scan tool before replacing the coolant temp sensor...I suspect that this may be the issue. One other thing though...If the air intake tube (after the MAF placement) is cracked or leaking, this will throw this code. Ensure no leakage is present.

Customer: replied 1 year ago.
I have changed the air flow sensor twice. One was a Toyota the other Duralast neither changed the problem. There was also a P1133 code Bank 1 sensor slow response that is why I changed the o2 sensor in bank 1. I don't understand why some times with the check engine on it runs great and other times it will hardly move. But I can reset with disconnecting the battery and it come back.
I will scan it for coolant temp and check for cracks. Does 2 trip detection mean the engine is stopped twice before it sets the code?
Expert:  Toyota Service replied 1 year ago.

Correct. This code is checked during*****cycles. When you disconnect the battery, you erase the stored short and long fuel trim calculations. That is why it resets.

Customer: replied 1 year ago.
It feels like it is stumbling like running out of gas. But this is only sometimes other times it just sets the code. Any thoughts on that.
Expert:  Toyota Service replied 1 year ago.

The MAF reads the air temperature and the air velocity (hopefully you do not have any aftermarket air filter; thsi is not good). The Coolant sensor measure just that. If the parameters are off, the fuel injection pulse duration (how long the computer squirts the fuel) may be backed off to one millisecond. This would cause a running out of gas feeling.

Customer: replied 1 year ago.
The coolant temp sensor and the gauge temp are two different sensors right? The gauge always show normal temp.
Expert:  Toyota Service replied 1 year ago.

Correct. 2 different sensors. The gauge sensor has nothing to do with fuel injection. It is for the gauge only

Customer: replied 1 year ago.
I couldnt readily test coolant temp so I changed the coolant temp sensor on the water outlet behind timing belt. This is also controlling the dash temp gauge, unplug it and dash gauge goes to cold. My Haynes manual says they are one and the same. Checked MAF housing and tubes no cracks or damage. Changed air filter also, no help.
I am clearing the codes by disconnecting battery. I went to autozone and someone there said it should be cleared with scanner, so they did but didn't help. P0171.
Expert:  Toyota Service replied 1 year ago.

I stand corrected. The coolant sensor sends the signal to the electronic control unit on this engine. There, it is split into sending the information to the gauge and the fuel calculation.

If the Coolant sensor is good, the rectification is puzzling.

This is the list of the areas to investigate right out of the book:

Air induction system
 Injector blockage
 Mass air flow meter
 Engine coolant temp. sensor
 Fuel pressure
 Gas leak on exhaust system
 Heated oxygen sensor (bank 1, 2 sensor 2)

You have replaced the MAF, the First exhaust sensor in the system and the coolant sensor.

The questions that remain are these:

Intake manifod gaskets (might be worth investigating)

Fuel pressure (I would not think this is your issue)

Injectors (SOme really good fuel injector cleaner would be recommended; use us a great product called BG 44-K

Possible exhaust leaks. (you did not mention this, so I would think there is none.)

At this point, I would add a can of BG 44-K to the tank and driver it, running the fuel down as quick as possible, then repeat. Clear the code and drive it, see if the code comes back. If that does not provide any joy, I would be looking at replacement of the intake manifold gaskets.

Expert:  Toyota Service replied 1 year ago.

I stand corrected. The coolant sensor sends the signal to the electronic control unit on this engine. There, it is split into sending the information to the gauge and the fuel calculation.

If the Coolant sensor is good, the rectification is puzzling.

This is the list of the areas to investigate right out of the book:

Air induction system
 Injector blockage
 Mass air flow meter
 Engine coolant temp. sensor
 Fuel pressure
 Gas leak on exhaust system
 Heated oxygen sensor (bank 1, 2 sensor 2)

You have replaced the MAF, the First exhaust sensor in the system and the coolant sensor.

The questions that remain are these:

Intake manifod gaskets (might be worth investigating)

Fuel pressure (I would not think this is your issue)

Injectors (SOme really good fuel injector cleaner would be recommended; use us a great product called BG 44-K

Possible exhaust leaks. (you did not mention this, so I would think there is none.)

At this point, I would add a can of BG 44-K to the tank and driver it, running the fuel down as quick as possible, then repeat. Clear the code and drive it, see if the code comes back. If that does not provide any joy, I would be looking at replacement of the intake manifold gaskets.

Expert:  Toyota Service replied 1 year ago.

I stand corrected. The coolant sensor sends the signal to the electronic control unit on this engine. There, it is split into sending the information to the gauge and the fuel calculation.

If the Coolant sensor is good, the rectification is puzzling.

This is the list of the areas to investigate right out of the book:

Air induction system
Injector blockage
Mass air flow meter
Engine coolant temp. sensor
Fuel pressure
Gas leak on exhaust system
Heated oxygen sensor (bank 1, 2 sensor 2)

You have replaced the MAF, the First exhaust sensor in the system and the coolant sensor.

The questions that remain are these:

Intake manifod gaskets (might be worth investigating)

Fuel pressure (I would not think this is your issue)

Injectors (SOme really good fuel injector cleaner would be recommended; use us a great product called BG 44-K

Possible exhaust leaks. (you did not mention this, so I would think there is none.)

At this point, I would add a can of BG 44-K to the tank and driver it, running the fuel down as quick as possible, then repeat. Clear the code and drive it, see if the code comes back. If that does not provide any joy, I would be looking at replacement of the intake manifold gaskets.

Customer: replied 12 months ago.
I replaced the o2 sensor with advice from a local Toyota dealer advising bank 1 was closest to bumper. Reading online I realize he was wrong so I changed the wrong one. I will be replacing the correct one hopefully tomorrow. After talking to several knowledgeable mechanics I believe it may be fuel injectors too possibly.
Expert:  Toyota Service replied 12 months ago.

Get some BG 44k and run it thru. This is some really good stuff. it will also clean any carbon buildup off of the valve surfaces.

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