2003 Chevy Impala PO121 WONT GO AWAY

2003 CHEVROLET IMPALA
78,500 MILES • 6 CYL • FWD • AUTOMATIC
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FAW484
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  • 6 POSTS
My car keeps getting a code of PO121. The rpm's will go up...it take's for ever for it to shift out of first gear..and when you push on the gas it wont go into passing gear..... its just dead. I have replaced the Throttle position sensor twice, replaced the idle air control sensor,and the speed sensor on the transmission. I wrote to one of your tech's about three weeks ago and this is what he told me:

start with the basics,check these first; Vacuum hoses disconnected, damaged, or incorrectly routed

� MAP sensor seal missing or damaged

� Restrictions in the MAP sensor vacuum source

� Intake manifold vacuum leaks

you also have to check for a 5 volt reference at the tps as well as see what the MAP sensor is doing,the PCM will use the manifold absolute pressure (MAP) sensor to determine if the predicted operating range of the TP sensor is correct. If the PCM detects that the TP sensor voltage is out of the predicted range DTC P0121 sets.start here first,post back with what you find.


I replaced the map sensor, replaced the pcv valve, the two o rings inside where the pcv valve is and the holder that holds the map sensor in place. And checked all the vacuum hose's. And it run good for two days and started with the bad running problems again and the same code of PO121.I have always cleared the code when i put a part on. I started with the easiest first since i have no ideal how to check for the voltage like the other tech said to do. And if I did..what would I do if the voltage was off. Since I've already replaced four sensors, i cant keep replacing sensors (because Im out of work) can you please help me with this. Thank You
Feb 21, 2010 at 7:21 PM
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BRIAN 1
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sorry I havent gotten back to you,follow this flow chart,Inspect for the following conditions:

• Any vacuum hoses disconnected, damaged, or incorrectly routed

• The manifold absolute pressure (MAP) sensor vacuum hose disconnected or damaged

• Restrictions in the MAP sensor vacuum source

• Any intake manifold vacuum leaks

Did you find and correct the condition?
--
Go to Step 24
Go to Step 3

3
Important: The vehicle used for the comparison is not limited to the same type of vehicle as is being serviced. A vehicle known to provide an accurate reading is acceptable.

Do you have access to another vehicle in which the MAP sensor pressure can be observed with a scan tool?
--
Go to Step 4
Go to Step 5

4
Turn ON the ignition, with the engine OFF.
Observe the MAP sensor pressure with a scan tool.
Observe the MAP sensor pressure in the known good vehicle with a scan tool.
Compare the values.
Is the difference between the values less than the specified value?
3 kPa
Go to Step 6
Go to Manifold Absolute Pressure Sensor Diagnosis

5
Important: The Altitude vs Barometric Pressure table indicates a pressure range for a given altitude under normal weather conditions. Weather conditions consisting of very low or very high pressure and/or very low or very high temperature may cause a reading to be slightly out of range.

Turn ON the ignition, with the engine OFF.
Observe the MAP sensor pressure with a scan tool. Refer to Altitude Versus Barometric Pressure .
The MAP sensor pressure should be within the range specified for your altitude.
Does the MAP sensor indicate the correct barometric pressure?
--
Go to Step 6
Go to Manifold Absolute Pressure Sensor Diagnosis

6
Observe the MAP sensor pressure with a scan tool.
Start the engine.
Does the MAP sensor pressure change?
--
Go to Step 7
Go to Manifold Absolute Pressure Sensor Diagnosis

7
Turn OFF the ignition.
Turn ON the ignition, with the engine OFF.
Observe the throttle position (TP) sensor parameter with the scan tool.
Is the voltage less than the specified value with the throttle in the closed position?
0.90 V
Go to Step 8
Go to Step 9

8
Observe the TP sensor parameter with the scan tool.
Depress the accelerator pedal slowly until the throttle is in the wide open position.
Release the accelerator pedal slowly until the throttle is returned to the closed position.
Repeat this procedure several times.
Does the TP sensor parameter increase steadily to more than the first specified value, and then decrease steadily returning to less than the second specified value?
98%

1%
Go to Intermittent Conditions
Go to Step 10

9
Inspect the throttle body for the following conditions:

• A binding or damaged throttle blade

• A binding or damaged throttle cable

Did you find and correct the condition?
--
Go to Step 24
Go to Step 10

10
Disconnect the TP sensor electrical connector.
Observe the TP sensor parameter with the scan tool.
Is the voltage less than the specified value?
0.1 V
Go to Step 11
Go to Step 15

11
Measure the voltage from the 5-volt reference circuit of the TP sensor to a good ground with a DMM. Refer to Circuit Testing in Wiring Systems.

Is the voltage more than the specified value?
5.2 V
Go to Step 16
Go to Step 12

12
Probe the 5-volt reference circuit of the TP sensor with a test lamp that is connected to a good ground. Refer to Circuit Testing and Probing Electrical Connectors in Wiring Systems.

Is the test lamp OFF?
--
Go to Step 17
Go to Step 13

13
Connect a 3-amp fused jumper wire between the 5-volt reference circuit of the TP sensor and the signal circuit of the TP sensor.
Observe the TP sensor parameter with the scan tool.
Is the voltage more than the specified value?
4.9 V
Go to Step 14
Go to Step 18

14
Turn OFF the ignition.
Remove the jumper wire.
Connect a jumper wire between each of the terminals in the TP sensor harness connector and the corresponding terminal at the TP sensor. Refer to Using Connector Test Adapters in Wiring Systems.
Turn ON the ignition, with the engine OFF.
Measure the voltage from the low reference circuit of the TP sensor at the jumper wire terminal to a good ground with a DMM. Refer to Measuring Voltage Drop in Wiring Systems.
Is the voltage more than the specified value?
0.2 V
Go to Step 19
Go to Step 20

15
Important: Disconnecting the powertrain control module (PCM) may eliminate the short during testing.

Test the TP sensor signal circuit between the PCM and the TP sensor for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 24
Go to Step 23

16
Important: Disconnecting the PCM may eliminate the short during testing.

Test all of the branches of the 5-volt reference circuit that are shared with the TP sensor for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 24
Go to Step 23

17
Test the 5-volt reference circuit between the PCM and the TP sensor for an open or for a high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 24
Go to Step 21

18
Test the TP sensor signal circuit between the PCM and the TP sensor for the following conditions:

• A short to ground

• An open

• A high resistance

Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 24
Go to Step 21

19
Test the low reference circuit between the PCM and the TP sensor for an open or for a high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 24
Go to Step 21

20
Test for an intermittent and for a poor connection at the TP sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 24
Go to Step 22

21
Test for an intermittent and for a poor connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?
--
Go to Step 24
Go to Step 23

22
Replace the TP sensor. Refer to Throttle Position Sensor Replacement .

Did you complete the replacement?
--
Go to Step 24
--

23
Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?
--
Go to Step 24
--

24
Clear the DTCs with a scan tool.
Turn OFF the ignition for 30 seconds.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.
Did the DTC fail this ignition?
--
Go to Step 2
Go to Step 25

25
Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?
--
Go to Diagnostic Trouble Code (DTC) List
System OK,post back with what you come up with.
Feb 23, 2010 at 6:27 PM