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westrope65, ASE Certified Technician
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99 silverado p0121-p0171-p0174-codes pop up soon after ...

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99 silverado p0121-p0171-p0174-codes pop up soon after start up.checked most critical is fuel pressure.runs great. bobbles at slight crack of throttle
Hi, welcome to

These codes are for throttle position sensor and lean condition.
121 for TPS
171 lean condition bank 1
174 lean condition bank 2

So look for vacumn leaks and possible intake leaks (manifold or lines)
it shows lean on both sides

hope this helps.______________
westrope65 and 4 other Car Specialists are ready to help you
Customer: replied 9 years ago.
mech-40yrs-plants-hotrods.AEM dry-BBK throttle bdy-granitelli maf-hypertech III-new TPS-have not found vac leak yet.runs like scalded dog.2-3gal better milage. sprayed rpm evap solonoid on canister.does act like vac leak.point me in right direction.did retighten intake bolts. all were tight.put orig. program in.went 120 mls before it went crazy those intakes crack.120,000 mls
Try looking again for the leaks on thei intake manifold when it is cold. Maybe yours is sealing better when it is warm.

We can try to test other components in the system and see if they are bad and not sending bad information.

Have you done anything with the O2 sensors? or anything elses for history?

The Throttle Position (TP) sensor is a potentiometer. The TP sensor is connected to the throttle shaft on the throttle body. The PCM calculates throttle position by monitoring the voltage on the signal line. The TP sensor signal changes as the throttle valve angle is changed (accelerator pedal moved). The TP sensor signal voltage is low at a closed throttle position. The TP sensor signal voltage increases as the throttle valve opens so that at Wide Open Throttle (WOT), the output voltage should be above 4.0 volts.

The PCM calculates fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor may cause intermittent bursts of fuel from an injector. This may cause an unstable idle because the PCM detects the throttle is moving.

The following DTCs set when the PCM detects a malfunction with the TP sensor circuits:
  • DTC P0121 TP Sensor Circuit Insufficient Activity.
  • DTC P0122 TP Sensor Circuit Low Voltage.
  • DTC P0123 TP Sensor Circuit High Voltage.
  • DTC P1121 TP Sensor CKT Intermittent High Voltage.
  • DTC P1122 TP Sensor CKT Intermittent Low Voltage.

hope this helps______________
Customer: replied 9 years ago.
can fuel pressure cause lite tocome on.
Yes if it was running lean. maybe fuel pressure regulator
can you get a testor? most parts stores will rent them cheap or free.

J 34730 - 1A fuel pressure guage Gauge

CAUTION: Refer to the Battery Disconnect Caution in Service Precautions.
  1. Disconnect the negative battery cable.
  2. Remove the engine sight shield.
  3. Relieve the fuel system pressure. Refer to the fuel pressure release Procedure.

  1. Disconnect the fuel pressure regulator vacuum line.
  2. Clean any dirt from the fuel pressure regulator retainer and the surrounding area.
  3. Remove the fuel pressure regulator retainer (8).
  4. Remove the fuel pressure regulator (9) from the fuel pressure regulator housing.
  1. Install the backup ring (10) on the fuel pressure regulator (9).
  2. Install the new large O-ring (11) on the fuel pressure regulator (9).
  3. Install the regulator filter (12) on the fuel pressure regulator (9).
  4. Install the new small O-ring (13) on the fuel pressure regulator (9).
  5. Lubricate the fuel pressure regulator large O-ring (11) and the small O-ring (13) with clean engine oil.
  6. Push the fuel pressure regulator into the regulator housing on the fuel line.
  7. Install a new fuel pressure regulator retainer (8).
  8. Connect the fuel pressure regulator vacuum line.

  1. Connect the negative battery cable.
  2. Tighten the fuel filler cap.
  3. Inspect for leaks.
11.1. Turn the ignition switch ON for 2 seconds.
11.2. Turn the ignition switch OFF for 10 seconds.
11.3. Turn the ignition switch ON.
11.4. Check for fuel leaks.
  1. Install the engine sight shield.
Circuit Description
When you turn ON the ignition switch, the Powertrain Control Module (PCM) turns ON the in-tank fuel pump. The in-tank fuel pump remains ON as long as the engine is cranking or running and the PCM receives reference pulses. If there are no reference pulses, the PCM turns the in-tank fuel pump OFF 2 seconds after the ignition switch is turned ON or 2 seconds after the engine stops running.

The electric fuel pump attaches to the fuel sender assembly inside the fuel tank. The in-tank fuel pump supplies fuel through an in-pipe fuel filter to the fuel rail assembly. The fuel pump provides fuel at a pressure above the pressure needed by the fuel injectors. A fuel pressure regulator, attached to the fuel rail, keeps the fuel available to the fuel injectors at a regulated pressure. Unused fuel returns to the fuel tank by a separate fuel return pipe.

Test Description
The numbers below refer to the step numbers on the diagnostic table.
  1. When the ignition switch is ON and the fuel pump is running, the fuel pressure indicated by the fuel pressure gauge should read 379-427 kPa (55-62 psi). The spring pressure inside the fuel pressure regulator controls the fuel pressure.
  2. A fuel system that drops more than 34 kPa (5 psi) in 10 minutes has a leak in one or more of the following areas:
    • The fuel pump check valve.
    • The fuel pump flex pipe.
    • The valve or valve seat within the fuel pressure regulator.
    • The fuel injector(s).
  1. A fuel system that drops more than 14 kPa (2 psi) in 10 minutes after being relieved to 69 kPa (10 psi) indicates a leaking fuel pump check valve.
  2. Fuel pressure that drops-off during acceleration, cruise or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. You can diagnose a lean condition using a scan tool. If an extremely lean condition occurs, the heated oxygen sensors(s) will stop toggling. The heated oxygen sensor output voltage(s) will drop below 300 mV. The fuel injector pulse width will increase.

Important: Make sure the fuel system is not operating in the Fuel Cut-Off Mode. This can cause false indications by the scan tool.

  1. When the engine is at idle, the manifold pressure is low (high vacuum). This low pressure (high vacuum) is applied to the fuel pressure regulator diaphragm. The low pressure (high vacuum) will onset the pressure being applied to the fuel pressure regulator diaphragm by the spring inside the fuel pressure regulator. When this happens, the result is lower fuel pressure. The fuel pressure at idle will vary slightly as the barometric pressure changes, but the fuel pressure at idle should always be less than the fuel pressure noted in step 2 with the engine OFF.
  1. A rich condition may result from the fuel pressure being above 427 kPa (62 psi). A rich condition may cause DTC P0132, DTC P0152, DTC P0172 or DTC P0175 to set. Driveability conditions associated with rich conditions can include hard starting followed by black smoke and a strong sulfur smell in the exhaust.
  2. This test determines if the high fuel pressure is due to a restricted fuel return pipe or if the high fuel pressure is due to a faulty fuel pressure regulator.
  1. A lean condition may result from the fuel pressure being below 379 kPa (55 psi). A lean condition may cause DTC P0131, DTC P0151, DTC P0171 or DTC P0174 to set. Driveability conditions associated with lean conditions can include hard starting (when the engine is cold), hesitation, poor driveability, lack of power, surging, and misfiring.

Notice: Do not allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel pressure regulator.

  1. Restricting the fuel return pipe with the J 37287 fuel pipe shut-off adapter causes the fuel pressure to rise above the regulated fuel pressure. Using a scan tool to pressurize the fuel system, the fuel pressure should rise above 427 kPa (62 psi) as the valve on the fuel pipe shut-on adapter connected to the fuel return pipe becomes partially closed.
  1. Check the spark plug associated with a particular fuel injector for fouling or saturation in order to determine if that particular fuel injector is leaking. If checking the spark plug associated with a particular fuel injector for fouling or saturation does not determine that a particular fuel injector is leaking, use the following procedure:
  1. Remove the fuel rail.
  2. Reinstall the crossover pipe to the right fuel rail.
  3. Connect the fuel feed pipe and the fuel return pipe to the fuel rail.
  4. Lift the fuel rail just enough to leave the fuel injector nozzles in the fuel injector ports.

Caution: In order to reduce the risk of fire and personal injury that may result from fuel spraying on the engine, verify that the fuel rail is positioned over the fuel injector ports. Also verify that the fuel injector retaining clips are intact.

  1. Pressurize the fuel system by using the scan tool fuel pump enable.
  2. Visually and physically inspect the fuel injector nozzles for leaks.