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Obd Code P0171


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#1 boeingdoc

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Posted 12 March 2010 - 07:57 AM

My 2005 Corolla S, automatic transmission, and 60,000 miles, has the check engine light on and the OBD scan shows code P0171 indicating system too lean. The car still runs well with no apparent troubles, i.e. idle smooth and acceleration normal. There are about 5 different things listed that could cause this. Does anyone have experience trouble shooting this problem? I don't see any vacuum leaks and I cleaned the MAF sensor. How can I tell if the MAF sensor is working correctly? How can I check the oxygen sensors?

#2 fishexpo101

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Posted 12 March 2010 - 10:18 AM

Did you reset the ECM after cleaning the MAF and still got a P0171 code come back? Both the O2 and MAF sensors can be checked for operation by backprobing the sensors while running the car. You could also run a datalogger to compare the values and make sure the ECM is reading the values correctly.

For a P0171 on a Toyota with a 1ZZ-FE engine - possible causes are:

- Leak in air induction system
- Faulty PCV hose
- Injector blockage
- Faulty mass air flow meter (MAF)
- Engine coolant temperature sensor
- Issue with fuel pressure (hard to check, need to use a SST from Toyota and T into the existing fuel line)
- Gas leakage on exhaust system
- Open or short in heated oxygen sensor (pre-cat) circuit
- Faulty heated oxygen sensor (pre-cat)
- Faulty ECM

#3 boeingdoc

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Posted 12 March 2010 - 11:47 AM



Did you reset the ECM after cleaning the MAF and still got a P0171 code come back? Both the O2 and MAF sensors can be checked for operation by backprobing the sensors while running the car. You could also run a datalogger to compare the values and make sure the ECM is reading the values correctly.

For a P0171 on a Toyota with a 1ZZ-FE engine - possible causes are:

- Leak in air induction system
- Faulty PCV hose
- Injector blockage
- Faulty mass air flow meter (MAF)
- Engine coolant temperature sensor
- Issue with fuel pressure (hard to check, need to use a SST from Toyota and T into the existing fuel line)
- Gas leakage on exhaust system
- Open or short in heated oxygen sensor (pre-cat) circuit
- Faulty heated oxygen sensor (pre-cat)
- Faulty ECM


OK. Thanks for the info. It looks like there are a few more things that I need to consider. No, I did not reset the ECM after cleaning the MAF. The little bulb hanging down in the MAF sensor was a little dirty, which I cleaned, but I didn't reset ECM. How would I do that? Disconnect / reconnect the battery? If I do back probe the MAF sensor, what voltage reading would be normal at idle and what should I see as the engine accelerates? Same question for the oxygen sensor - what would normal readings be? When I had the codes read at Auto Zone the print out also mentions the oxygen sensor after the cat but I see you don't list that. Are you suggesting that if an oxygen sensor is causing the problem, it would only be the pre-cat sensor and not the post-cat sensor?

#4 fishexpo101

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Posted 12 March 2010 - 12:39 PM

Disconnecting the battery would be a start - negative battery terminal off for a minute - might help to step on the brake or flip on the headlamps to drain any residual charge. Some cars cannot be reset in this manner - have to be reset with an OBD-II scanner. This is more likely in cases where CAN bus is being used - like in your model year Corolla. Keep in mind that anything that depends on power will also be affected - radio presets, some headunits have builtin security, alarm systems, etc. If in doubt - just pickup a CAN compliant OBD-II scanner at an autoparts store. They are really getting inexpensive and just pulling and reseting a single CEL - it will pay for itself (Toyota dealership's typically charge around $85 for a diagnostic fee).

The little bulb thing is the IAT sensor (intake air sensor) - the MAF sensor itself is inside the plastic cylinder MAF housing (should be able to make out two "wires" on the inside of the tube). Backprobing the sensor - you should see a rise in voltage as more air flows over those MAF sensor - so as you increase the throttle - the voltage will increase from the MAF. Same with the O2 sensors, you should be able to see the voltage trace (o-scope is best here) and view the waveform in real-time. Yours will be a little different than conventional O2 sensors, as the 2005+ Corolla models now use an air/fuel ratio sensor instead of an O2 sensor. The sensor is much more sensitive and has a wider sensing band. In other words, the new sensor acts more like a wide-band O2 sensor (read expensive) compared to the garden variety narrow-band O2 sensor.

The after cat O2 sensor on the car may flag a code if the heater circuit malfunctions, but generally not considered an issue for a P0171. The only job the rear O2 sensor does is monitor how effectively the catalytic converter is doing its job. The air/fuel mix is determined by the upstream or pre-cat sensor.

My suspicion is that once the CEL is reset and the MAF was cleaned carefully - the code should drop away, no more CEL.

#5 boeingdoc

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Posted 12 March 2010 - 02:12 PM

Disconnecting the battery would be a start - negative battery terminal off for a minute - might help to step on the brake or flip on the headlamps to drain any residual charge. Some cars cannot be reset in this manner - have to be reset with an OBD-II scanner. This is more likely in cases where CAN bus is being used - like in your model year Corolla. Keep in mind that anything that depends on power will also be affected - radio presets, some headunits have builtin security, alarm systems, etc. If in doubt - just pickup a CAN compliant OBD-II scanner at an autoparts store. They are really getting inexpensive and just pulling and reseting a single CEL - it will pay for itself (Toyota dealership's typically charge around $85 for a diagnostic fee).

The little bulb thing is the IAT sensor (intake air sensor) - the MAF sensor itself is inside the plastic cylinder MAF housing (should be able to make out two "wires" on the inside of the tube). Backprobing the sensor - you should see a rise in voltage as more air flows over those MAF sensor - so as you increase the throttle - the voltage will increase from the MAF. Same with the O2 sensors, you should be able to see the voltage trace (o-scope is best here) and view the waveform in real-time. Yours will be a little different than conventional O2 sensors, as the 2005+ Corolla models now use an air/fuel ratio sensor instead of an O2 sensor. The sensor is much more sensitive and has a wider sensing band. In other words, the new sensor acts more like a wide-band O2 sensor (read expensive) compared to the garden variety narrow-band O2 sensor.

The after cat O2 sensor on the car may flag a code if the heater circuit malfunctions, but generally not considered an issue for a P0171. The only job the rear O2 sensor does is monitor how effectively the catalytic converter is doing its job. The air/fuel mix is determined by the upstream or pre-cat sensor.

My suspicion is that once the CEL is reset and the MAF was cleaned carefully - the code should drop away, no more CEL.



Thanks again for your insight and helpful suggestions. I'm away from the car right now but I will look into it further over the weekend and post the results Monday. Thanks again.

#6 dshadle1

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Posted 13 March 2010 - 08:46 PM

Thanks again for your insight and helpful suggestions. I'm away from the car right now but I will look into it further over the weekend and post the results Monday. Thanks again.


I had a p0171 intermittently for years until I cleaned the MAF. Now I do it every time the light comes on (usually once a year as soon as the weather turns hot). Never had problems since I started cleaning it regularly.

Now, however, I'm fighting an exhaust leak from the donut gasket between the exhaust manifold and the exhaust downpipe--right before the front oxygen sensor. P0171 stayed on even after cleaning the MAF (we had 70 degrees last week), so it is almost certainly the exhaust leak causing it.

Moral of the story is that if you are still getting the code after thoroughly cleaning the MAF, cleaning it again, and then resetting by disconnecting battery, there is an OK chance that there's a leak at that point in the exhaust. Those gaskets fail all the time. You can take it to a few places like Meineke for a free exhaust test if you need to go that route. Much cheaper than paying a mechanic to diagnose the more complex causes of the code and only to find out it's an exhaust leak before the o2 sensor.

If neither of these works, I would move on to intake/vacuum leak. Also pretty easy to test at home with a spray bottle.

Let us know what works!

#7 The Happy Idiot

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Posted 16 August 2010 - 11:30 AM

I'm having the same problem with my 2000 corolla. Cleaned the MAF, cleared the code, drove ~40 miles and got the 0171 again. Biggest problem is my NYS inspection running out and now just clearing the code and heading down to the repair shop for the inspection doesn't cut it. I plan on putting in new spring bolts and gasket between the manifold and the frontpipe as well as replacing the front o2 sensor. A new Denso OEM sensor from rockauto.com was 68 bucks so I don't know why people are saying they're so expensive. Maybe 'cuz it's not an air/fuel ration jobby?

My question is, do I really need a special tool for the spring bolts?

#8 dshadle1

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Posted 16 August 2010 - 02:59 PM

I'm having the same problem with my 2000 corolla. Cleaned the MAF, cleared the code, drove ~40 miles and got the 0171 again. Biggest problem is my NYS inspection running out and now just clearing the code and heading down to the repair shop for the inspection doesn't cut it. I plan on putting in new spring bolts and gasket between the manifold and the frontpipe as well as replacing the front o2 sensor. A new Denso OEM sensor from rockauto.com was 68 bucks so I don't know why people are saying they're so expensive. Maybe 'cuz it's not an air/fuel ration jobby?

My question is, do I really need a special tool for the spring bolts?


Definitely not. I got mine on and off--albeit with some difficulty and stripped threads--with just a regular socket+ratchet+extension. I attribute the stripping to not being careful about trying to stay 90 degrees to the bolt-spring.

#9 The Happy Idiot

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Posted 16 August 2010 - 08:48 PM


I'm having the same problem with my 2000 corolla. Cleaned the MAF, cleared the code, drove ~40 miles and got the 0171 again. Biggest problem is my NYS inspection running out and now just clearing the code and heading down to the repair shop for the inspection doesn't cut it. I plan on putting in new spring bolts and gasket between the manifold and the frontpipe as well as replacing the front o2 sensor. A new Denso OEM sensor from rockauto.com was 68 bucks so I don't know why people are saying they're so expensive. Maybe 'cuz it's not an air/fuel ration jobby?

My question is, do I really need a special tool for the spring bolts?


Definitely not. I got mine on and off--albeit with some difficulty and stripped threads--with just a regular socket+ratchet+extension. I attribute the stripping to not being careful about trying to stay 90 degrees to the bolt-spring.


I'll give it a shot. Any clue as to the size of the nuts on the heat shield for the manifold? At 180K miles, I'm certain I'll be turning them into powder. I like to have those replacements before I do the job, ya know?

#10 dshadle1

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Posted 17 August 2010 - 06:20 PM



I'm having the same problem with my 2000 corolla. Cleaned the MAF, cleared the code, drove ~40 miles and got the 0171 again. Biggest problem is my NYS inspection running out and now just clearing the code and heading down to the repair shop for the inspection doesn't cut it. I plan on putting in new spring bolts and gasket between the manifold and the frontpipe as well as replacing the front o2 sensor. A new Denso OEM sensor from rockauto.com was 68 bucks so I don't know why people are saying they're so expensive. Maybe 'cuz it's not an air/fuel ration jobby?

My question is, do I really need a special tool for the spring bolts?


Definitely not. I got mine on and off--albeit with some difficulty and stripped threads--with just a regular socket+ratchet+extension. I attribute the stripping to not being careful about trying to stay 90 degrees to the bolt-spring.


I'll give it a shot. Any clue as to the size of the nuts on the heat shield for the manifold? At 180K miles, I'm certain I'll be turning them into powder. I like to have those replacements before I do the job, ya know?


Do you mean bolts? If so, 12 mm I believe. Mine were pretty rusted but stayed intact...but I had 100k fewer miles than you the last time I took them off! For the one on the bottom of the passenger side, it's easier if you go with your left hand. Takes a little bit of working it to get it in and out.

Edited by dshadle1, 17 August 2010 - 06:21 PM.

#11 The Happy Idiot

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Posted 20 August 2010 - 09:32 PM

Got the heat shield off without too much issue. The nuts are 10mm, by the way. One of the studs crumbled away but it was the bottom one and the other two hold in place well enough. I replaced the o2 sensor with a Denso OEM. I stripped one of the nuts using a 12-pt socket. Luckily, I had bought replacements for my Tacoma when I replaced the muffler but, wouldn't ya know, JBA included stainless nuts with the cat-back system. The ones I bought are the same size as the Corolla (Thank you Toyota engineers). I also replaced the spring bolts and exhaust gasket there, though the old one didn't look too bad. Really, the whole job was pretty easy and didn't require too much squeezing of the hands into tight spaces or snub ratchets or anything. Just some extensions and PB Blaster. We'll have to see if the CEL comes back on with the p0171. Gotta get the thing inspected.

Edited by The Happy Idiot, 20 August 2010 - 09:34 PM.

#12 The Happy Idiot

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Posted 22 August 2010 - 08:37 PM

Blast! CEL is back on. Gave me a p0171 again so I re-cleaned the MAF sensor, cleaned the PCV valve and cleared the code. After about 80 miles CEL comes back on. This time it's a p0420. I don't have access to a data logger I'd like to back probe the o2 sensors but my search of the forums hasn't turned anything up. I'd love to find out I don't need a new converter. Any tips?

#13 The Happy Idiot

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Posted 22 August 2010 - 08:41 PM

Okay, after digging a bit more I found, of course, fishexpo101's explanation and helpful diagrams. Thanks again! I'll be tackling this soon....

#14 dshadle1

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Posted 24 August 2010 - 07:23 PM

Okay, after digging a bit more I found, of course, fishexpo101's explanation and helpful diagrams. Thanks again! I'll be tackling this soon....


Definitely could be the post-cat sensor acting finicky.

#15 The Happy Idiot

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Posted 29 August 2010 - 07:49 PM

I ended up just buying a new sensor rather than messing with checking it. I'm running out of time for the inspection. I haven't gotten the part yet. I hope it'll get rid of the p0420 but I still have to deal with the p0440, p0441 and p0446. Yeesh!