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P0121 Code On 2008 Silverado Help!


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#1 Daytona955

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Posted 03 June 2009 - 07:48 AM

OK, heres the deal. 2008 Silverado ext cab 5.3 with 6k miles on the clock.

Lately the MIL lamp has been coming on for a few days then going for a day or two then back on again etc.

So I get a scan unit via ebay ( I'm living in one of the Gulf Arab states) and it shows code P0121. A search tells me this is a throttle position sensor/Switch A Circuit Range/Performance Problem.

So I clear the code and the next day after 3 starts the light is back on again. :D

I don't want to take the truck to the one and only GM dealer in town as they had it for over a week just for the first service and would have had it longer if I hadn't shown up and demanded it's return. Plus the standard of their workmanship and care leaves alot to be desired.

The only mod's i've fitted is a K&N CAI and a Black Bear tune.

Further research has had one guy on another forum claim to have removed his TPS and clean a bunch of oil out of the recess in which it sits which fixed the problem. Only problem is, I don't know where the TPS even is on a 5.3 Vortec. I'm kinda handy with tools and such so theres no problem with tackling the issue myself once I get some advice on where to look and what to remove etc.

Considering the simple mod's I've fitted is it possible either of them may be causing the problem?

There seems to be no other issues like stumbling on idle or acceleration so the only issue at this stage is the inability to use the remote start when the MIL is on which really sucks during these days when the temps are hitting 118 and we aren't even totally into summer yet.

The scan unit I got was fairly basic so I won't be able to check the voltage from the PCM at idle and WOT.

Does anyone have any suggestions?

#2 swhitney2003

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Posted 03 June 2009 - 07:55 AM

From service manual:

DTC P0068 or P0121
Diagnostic Instructions
Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Review Strategy Based Diagnosis for an overview of the diagnostic approach.

Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors
DTC P0068: Throttle Body Airflow Performance

DTC P0121: Throttle Position (TP) Sensor 1 Performance


Circuit/System Description
The engine control module (ECM) uses the following information to calculate an expected airflow rate:

The throttle position (TP)

The barometric pressure (BARO)

The manifold absolute pressure (MAP)

The intake air temperature (IAT)

The engine RPM

Conditions for Running the DTC
P0068
DTCs P0641, P0651, P1516, P2101, P2119, P2176 are not set.

The engine is running.

DTC P0068 run continuously when the above conditions are met.

P0121
DTCs P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0128, P0315, P0335, P0336 are not set.

The engine speed is more than 450 RPM.

DTC P0121 run continuously when the above conditions are met.

Conditions for Setting the DTC
P0068
The ECM detects that the throttle position and the indicated engine load do not correspond with the expected load and throttle position for less than 1 second.

P0121
The predicted air flow and the predicted MAP combined are outside a calibrated range for more than 3 seconds.

Action Taken When the DTC Sets
P0068
DTC P0068 is a Type A DTC.

The control module commands the throttle actuator control (TAC) system to operate in the Reduced Engine Power mode.

A message center or an indicator displays Reduced Engine Power.

Under certain conditions the control module commands the engine OFF.

P0121
DTC P0121 is a Type B DTC.

Conditions for Clearing the MIL/DTC
DTC P0068 is a Type A DTC.

DTC P0121 is a Type B DTC.

Reference Information
Schematic Reference
Engine Controls Schematics

Connector End View Reference
Component Connector End Views

Electrical Information Reference

Circuit Testing

Connector Repairs

Testing for Intermittent Conditions and Poor Connections

Wiring Repairs


DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference
Control Module References for scan tool information

Circuit/System Testing
Inspect for the following conditions:
Vacuum hoses for splits, kinks, and proper connections as shown on Vehicle Emission Control Information label

Inspect thoroughly for any type of leak or restriction

Air leaks at throttle body mounting area and intake manifold sealing surfaces

Throttle body for dirt, debris, and coking--Refer to Throttle Body Cleaning.

Allow the engine to reach operating temperature. Observe the MAP Sensor Voltage parameter with a scan tool. Voltage should be more than 0.8 volt and less than 4 volts.
⇒ If more than 4 volts or less than 0.8 volt, refer to DTC P0106.

Idle the engine. Observe the MAP Sensor kPa parameter with a scan tool. Increase the engine speed slowly and then back to idle. The MAP sensor kPa should change smoothly and gradually as engine speed is increased and returned to idle.
⇒ If the MAP sensor kPa does not change, refer to DTC P0106.

Take a snapshot of the engine data list while performing the actions listed below. The mass air flow (MAF) sensor g/s should change smoothly and gradually as the engine speed is increased and is returned to idle.
Idle the engine.

Increase the engine speed slowly to 3,000 RPM, then back to idle.

Exit from the snapshot and review the data.

Observe the Mass Air Flow (MAF) Sensor parameter frame by frame with a scan tool. The MAF sensor g/s should change smoothly and gradually as the engine speed is increased and is returned to idle.

⇒ If the MAF sensor g/s does not change smoothly and gradually as the engine speed is increased and is returned to idle, refer to DTC P0101 or P1101.

Inspect the throttle body for the following conditions:
Loose or damaged throttle blade

Broken throttle shaft

Any throttle body damage

⇒ If any of these conditions exist, replace the throttle body assembly.


2008 Silverado 1500 LT1 - Regular Cab 8' Bed - 5.3L, 3.73, 4x4, Z71 - Line-X Spray-In Bed Liner, 2" Rough Country Leveling Kit, DIC Install, Corsa dB Black Muffler, Kobalt Truck Chest, Silverstar Ultra Hi/Low, Audiobahn A8000V Amplifier - 800 Watts RMS @ 2Ω, Two 12" Audiobahn Subwoofers, Antenna removed, Air dam removed

#3 swhitney2003

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Posted 03 June 2009 - 07:57 AM

My best guess is with the K&N air filter. Too much oil maybe? Can you put the stock air intake on for a couple of days and see how that works?
2008 Silverado 1500 LT1 - Regular Cab 8' Bed - 5.3L, 3.73, 4x4, Z71 - Line-X Spray-In Bed Liner, 2" Rough Country Leveling Kit, DIC Install, Corsa dB Black Muffler, Kobalt Truck Chest, Silverstar Ultra Hi/Low, Audiobahn A8000V Amplifier - 800 Watts RMS @ 2Ω, Two 12" Audiobahn Subwoofers, Antenna removed, Air dam removed

#4 Daytona955

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Posted 03 June 2009 - 02:33 PM

My best guess is with the K&N air filter. Too much oil maybe? Can you put the stock air intake on for a couple of days and see how that works?



Yeah I could put the stock intake back on but I doubt thats the problem. I had the K&N on for maybe 4 months before the warning light came on. Is K&N known for over oiling their air filters? I hadn't heard that if they are...

If the filter has been over oiled I guess its the MAF sensor that needs to be cleaned? Aren't these things pretty sensitive? How exactly would I go about cleaning it safely?

#5 Daytona955

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Posted 05 June 2009 - 05:21 AM

Now this is getting annoying, I seem to get maybe 3 or 4 starts after each code clear before the light comes back on again.

Has anyone else has a P0121 DTC and what did you do to fix it?

#6 Draenor

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Posted 05 June 2009 - 08:08 AM

I would pull the TB off and liberally clean out the back side of it . Also, since you put in a CAI, I'd double check the wiring and connections to the sensors.


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#7 Triki Dick

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Posted 21 November 2011 - 09:48 AM

I know this is a stupid question, and I've searched this,

Can someone please take a photo of there Throttle body, so I know which part I need to clean.

Sorry for the stupid question!

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#8 txab

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Posted 21 November 2011 - 11:59 AM

I know this is a stupid question, and I've searched this,

Can someone please take a photo of there Throttle body, so I know which part I need to clean.

Sorry for the stupid question!


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