P1128 Bank 1 Upstream, P1129 Bank 2 Upstream Closed Loop Fueling not Achieved

This diagnosis for code:
- P1128 Closed Loop Fueling not Achieved - Bank 1 Upstream
- P1129 Closed Loop Fueling not Achieved - Bank 2 Upstream


P1128, P1129 code - This diagnostic runs when the following condition are met, shortly after engine start up, fuel level above 12%, battery voltage above 11. volts, no active bank 1 fuel injector, ignition coil, or upstream O2 sensor circuit faults.



Set condition, once the enable time has been reached and enable condition are met, the diagnostic will check for close loop fuel control. If close loop control is not active by the time the enable timer has expired, the diagnostic will fail. Defaul action, MIL light is illuminate.

Possible causes P1128 and P1129 code

- Rectricted fuel supply line
- Fuel pump inlet strainer plugged
- Fuel pump module
- O2 sensor heater operation
- O2 sensor signal circuit
- O2 sensor circuit
- O2 sensor
- ECT sensor operation
- Engine Mechanical Problem
- Fuel filter/pressure regulator
- Powertrain Control Module (PCM)

Diagnostic test P1128 and P1128 code

Step 1. ECT sensor operation
Diagnose and repair any other O2 sensor DTCs that are present before continuing with test procedure. This test works best if perfromed on a cold engine (cold soak).
Turn the ignition on, with the scan tool, read the engine cooland temperature (ECT) sensor value. If the engine was allowed to sit overnight (cold soak), the temperature valeu should be a sensible value that is somewhare close to be ambient temperature. Compare the ECT sensor value with the IAT sensor and AAT sensor values. If engine coolant temperature is above 82°C (180°F), allow the engine to cool until 65°C (150°F) is reached. Start the engine and allow it to idle, during engine warm-up, monitor the ECT sensor value. The temperature value change should be a smooth transition from start up to normal operating temperature 82°C (180°F). The value should reach at least 82°C (180°F).

Is the ECT Sensor value within a reasonable range with the other sensors at ignition on and did the ECT value increase smoothly and did it reach at least 82°C (180°F)?

Yes, go to step 2.
No, verify that there is good pin to terminal contact in the engine coolant temperature sensor and powertrain Control Module (PCM). If ok, replace the Engine coolant temperature.

Step 2. Check for an active DTC
With the scan tool read DTCs and record on the repair order. test drive or operate the vehicle.

Is the DTC active or pending?

Yes, go to step 3.
No, perform the PCM intermittent condition.

Step 3. Check te fuel delivery system
Perform the checking the fuel delivery system.

Were any problems found?

Yes, perform the appropriate procedure.
No, go to step 4.

Step 4. O2 sensor heater operation
Turn the ignition off. Wait a minimum of 10 minutes to allow the O2 sensor to cool down before continuing the test. Allow the sensor voltage to stabilize at 5.0 volts. Turn the ignition on with the scan tool actuate the sensor 1/1 heater test (for P1128 code) and 2/1 heater test (for P1129 code). With the scan tool monitor O2 sensor 1/1 (for P1128 code) and 2/1 (for P1129 code) voltage for at least two minutes.

Does the voltage stay above 4.5 volts?

Yes, verify that there is good pin terminal contact in the O2 sensor and Powertraint Control Module connectors. If problems  are found, replace the O2 sensor.
No, go to step 5.

Step 5. Check the O2 sensor operation
Star the engine, with the scan tool monitor the O2 sensor 1/1 (for P1128 code) and O2 sensor 2/1 (for P1129 code) reading.
If one sensor of the O2 sensor signal or return circuit is shorted to ground the scan tool will display all O2 senspr voltage low. O2 sensor that is shorted to ground will display a voltage reading near or at 0 volts. If one of the O2 sensor signal or return circuits are shorted to voltage, the scan tool will display all O2 sensor voltage readings high.

Is the voltage switching between 2.5 and 3.4 volts?

Yes, go to step 8.
No, go to step 6.

Step 6. Check the upstream O2 sensor return circuit
Turn the ignition on, engine not running, disconnect the O2 sensor 1/1 (for P1128 code) and O2 sensor 2/1 (for P1129 code) harness connector. Measure the voltage on the O2 sensor return circuit in the O2 sensor harness connector.

Is the voltage at 2.5 volts?

Yes, go to step 7.
No, check the O2 sensor 1/1 (for P1128 code) and O2 sensor 2/1 (for P1129 code) return circuit for a short to ground, open or short to voltage and repair if necessary. Inspect the O2 sensor 1/1 (for P1128 code) and O2 sensor 2/1 (for P1129 code) connector and the PCM connector. If no problems are found, replace and program the Powertrain Control Module (PCM). Perform the Powertrain Verification Test.

Step 7. Check the O2 sensor 1/1 (for P1128 code) and O2 sensor 2/1 (for P1129 code) signal circuit 
With the scan tool monitor the O2 sensor 1/1 (for P1128 code) and O2 sensor 2/1 (for P1129 code) voltage. O2 sensor should read between 4.1 and 5.0 volts on the scan tool with the connector  disconnected. Connect a jumper between the O2 sensor 1/1 (for P1128 code) and O2 sensor 2/1 (for P1129 code) signal circuit and the O2 sensor return circuit at the O2 sensor 1/1 (for P1128 code) and O2 sensor 2/1 (for P1129 code) harness connector. The voltage should drop from between 4.1 and 5.0 volts with the jumper in place.

Did the O2 sensor voltage change from between 4.1 and 5.0 volts with the jumper installed?

Yes, verify that there is good pin to terminal contact in the O2 sensor and Powertrain Control Module connectors. if no problems are found, replace the O2 sensor.

No, check the O2 sensor 1/1 (for P1128 code) and O2 sensor 2/1 (for P1129 code) signal circuit for a short to ground, open or short to voltage and repair if necessary. Inspect the O2 sensor 1/1 (for P1128 code) and O2 sensor 2/1 (for P1129 code) connector and the PCM harness connector. If no problems are found, go to step 9.

Step 8. Engine mechanical problem
Turn the ignition off, check for any of the following conditions/mechanical problems:
  • Air induction system - must be free from leaks
  • Engine vacum - must be at least 13 inches in neutral 
  • Engine valve timing - must be within specifications
  • Engine compression - must be within specifications
  • Engine exhaust system - must be free of any restrictions or leaks
  • Engine PCV system - must flow freely
  • Torque converter stall speed - must be within specifications
  • Power brake booster - no internal vacuums leaks
  • Fuel - must be free contamination
  • Fuel injector - plugged or restricted injector, control wire not connected to correct injector
Are there any engine mechanical problems?

Yes, repair as necessary.
No, go to step 9.

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