Manufacturer error Code 1271?

Hud477 said:

I’ve seen a few different ways to solve this but still confused. Had it go into check engine and go into reduced power mode 2 times in the last 2 weeks. I cleared the codes and it seems to be running fine. Any idea what could be causing this? It’s a 2002 TB with 160k.

Not sure if it matters, but cleaned throttle body about 10k ago. Last month did new water pump fan clutch and coolant temp sensor.

Any advice appreciated. Was planning on taking wife and 1 year old on beach with it in 2 weeks, but a bit worried now.

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This code is for the APP (accelerator pedal position) sensors. Essentially the code sets when the two of these sensors don't agree about where the accelerator pedal is positioned or one or both of the sensors is reporting an invalid value.

Since the two sensors use different 5 volt reference power supplies if one of those 2 power supplies has an issue like an intermittent short to ground or a high load it can affect the pedal sensor readings.

It is often suggested to disconnect the fan clutch to see if the issue goes away. This is done because the fan clutch uses one of the same power supplies as one of the APP sensors. But if the issue is infrequent this is not likely to be a good option since one should not just drive around for a long time without a working cooling fan.

When I face something like this I capture live data for the systems involved and hope to catch the issue in the act in the recorded data so I know better what to look for. With something that only happens once a week or so that can take quite a bit of time though.

Some will suggest to replace the APP sensor assembly. If money is no concern this may (or may not) help.

Unfortunately there is no simple quick fix for this.

#PIP3461: Information on Normal APP and TP Signal Circuit 2 Voltage Fluctuation - keyword DTC low MIL P0122 P0123 P0220 P0222 P01120 P1220 P1221 P1271 P1275 P1280 P1512 P1514 P1515 P1523 P2101 P2120 P2122 P2123 - (Jun 8, 2005)


Subject: Information on Normal APP and TP Signal Circuit 2 Voltage Fluctuation


Models: 2004-2006 Buick Rainier

2005-2006 Chevrolet Cobalt

2004-2006 Chevrolet Colorado

2002-2006 Chevrolet TrailBlazer

2004-2006 GMC Canyon

2002-2006 GMC Envoy

2006 Hummer H3

2002-2004 Oldsmobile Bravada

2005-2006 Pontiac Pursuit (Canada Only)

2005-2006 Saab 9-7X

2005-2006 Saturn Ion

2004-2006 Chevrolet Malibu

with 2.2L Engine (VIN F - RPO L61)

2002-2006 Saturn Vue

with 2.2L Engine (VIN D - RPO L61)




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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern:
If the APP sensor 2 or TP sensor 2 signal circuit is monitored with an oscilloscope or Fluke 87 DVOM on the 1 ms min/max setting, it may be noted that the signal voltage decreases every 156 ms for 6.25 - 7.8 ms each time.

Recommendation/Instructions:
It is normal for these voltages to drop as described above. This is the result of the PCM testing the APP and TP sensors for internal shorts. During the PCM test for a DTC P2135 (P1221 on 02-03 Vue), the PCM shorts the TP sensor 2 signal to ground every 156 ms for 6.25 - 7.8 ms each time. During the PCM test for a DTC P2138 (P1271 on 02-03 Vue), the PCM shorts the APP sensor 2 signal to ground every 156 ms for 6.25 - 7.8 ms each time. At this time, the PCM monitors the APP sensor 1 and TP sensor 1 signal circuit voltages to ensure that they do not decrease with the sensor 2 signal circuits. If they do, an internal sensor short is present or the signal circuits are shorted together.

If there are no DTCs resetting but toggling voltages are encountered during inspection with an oscilloscope or Fluke 87 DVOM on the 1 ms min/max setting, no repairs should be attempted to prevent the voltages from toggling as it is a normal condition. If DTCs are resetting, follow the related SI diagnostic charts to diagnose the ETC system.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

Hud477 said:

Thanks, I do have OBD link, just haven’t used it for tracking live data. The only other thing I’ve noticed is it does seem to happen shortly after I’ve done a quick acceleration like pulling out on a main road.

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Here is a look at what is possible with that OBD Link and an app called Car Scanner ELM OBD2. The app has the ability to record data as you drive for later review. The recording and display is all done within the app. In this way you are not distracted while driving. If you can manage to be recording when the fault occurs the data record can help to pinpoint what is causing the trouble.

In the app, after setting up your vehicle profile and adding in manufacturer specific parameters, you enable data recording and then select live data and select the parameters you want recorded. Then just drive.

Here I have recorded the sensors for the accelerator pedal and the sensors for throttle position, each displayed as a percentage. I also have access to the data displayed as a voltage. This recording shows there will be small fluctuations between the pairs of sensors. The fault you are getting indicates that your two accelerator pedal position sensors either exceed the allowable difference or one (or both) of the sensors are occasionally reporting an 'out of range' value that is either too high or too low a value.



If you care to try this I can assist with the setup, particularly with the importing of TrailBlazer-specific data parameters.

FTR ---> I can "see" the self-test glitches on my oscilloscope - and they appear as normal, per your statements too.

The Hi-Lo to Lo-Hi pot values are a mirror image of each other.

I've been haphazardly chasing this REP for all the years I've owned this '05 TB EXT 4.2, and have found some of the strangest things .....