Found this for the P2155 code.
heres the diagnoses from GM's service information for ya. when it comes to checking resistance in the injector dont forget to check from pin to housing of injector, it is possible it is shorted internally
Circuit/System Description
The engine control module (ECM) supplies voltage to each fuel injector on the injector positive voltage control circuits. The ECM energizes each fuel injector by grounding the control circuit of that fuel injector. The ECM monitors the status of the injector positive voltage control circuits and the fuel injector control circuits. The injectors are separated into the following four groups:
•Group 1–DTC P2146 with injectors 1 and 4
•Group 2–DTC P2149 with injectors 6 and 7
•Group 3–DTC P2152 with injectors 2 and 5
•Group 4–DTC P2155 with injectors 3 and 8
When a fuel injector circuit condition is detected by the ECM, the fuel injectors in the affected group will be disabled and set the group DTC and may also set the fuel injectors DTCs.
Conditions for Running the DTC
•The ignition is ON.
•The charging system voltage is between 6–18 volts.
Conditions for Setting the DTC
•The ECM injector output driver detects an incorrect current on a fuel injector circuit.
•The above condition is present for 3 seconds for groups 1, 2, and 3.
•The above condition is present for 5 seconds for group 4.
Action Taken When the DTC Sets
•The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
•The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records.
Conditions for Clearing the MIL/DTC
•The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
•A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
•A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
•Clear the MIL and the DTC with a scan tool.
Reference Information
Schematic Reference
Engine Controls Schematics
Connector End View Reference
Engine Control Module Connector End Views
Electrical Information Reference
•Circuit Testing
•Connector Repairs
•Testing for Intermittent Conditions and Poor Connections
•Wiring Repairs
Circuit/System Verification
Important: If you cannot duplicate the condition, 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.
1. Observe the Balancing Rate parameters with a scan tool. The normal reading should be between +4 mm³ and −4 mm³ at idle, and between +6 mm³ and − 6 mm³ in drive. With the engine running at idle, the Actual Fuel Rail Pressure should be close to the Desired Fuel Rail Pressure.
2. Engine running, observe the DTC information with a scan tool. DTCs P2146, P2149, P2152, or P2155 should not set.
3. 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 data.
Circuit/System Testing
Fuel Injector Group DTC set
1. Turn OFF the ignition. Allow 60 seconds for the ECM to power down.
2. Disconnect the ECM connector.
3. Connect a test lamp between the fuel injector positive voltage control circuit and battery voltage.
⇒ If the test lamp illuminates, test the fuel injector positive voltage circuit for a short to ground.
4. Test the fuel injector positive voltage control circuit and the fuel injector control circuit for high resistance.
5. Test the affected fuel injector for high resistance. Refer to Component Testing in this diagnostic.
⇒ If all circuits test normal, replace the ECM.
Fuel Injector Group DTC and Fuel Injector DTCs set
1. Turn OFF the ignition. Allow 60 seconds for the ECM to power down.
2. Disconnect the ECM connector.
3. Connect a test lamp between the fuel injector positive voltage control circuit and a ground.
⇒ If the test lamp illuminates, test the fuel injector positive voltage control circuit for a short to voltage.
4. Connect a test lamp between the affected fuel injector control circuit and battery voltage.
⇒ If the test lamp illuminates, test both fuel injector control circuits of the affected group for a short to ground.
5. Test the affected fuel injector for high resistance. Refer to Component Testing in this diagnostic.
⇒ If all circuits test normal, replace the ECM.
Component Testing
1. Disconnect the connector of the fuel injector to be tested.
2. Connect a DMM between the terminals of the fuel injector and measure the resistance. The resistance should be 0.2–0.3 ohm.
⇒ If resistance is more than 0.2–0.3 ohm resistance, replace the fuel injector.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
•Fuel Injector Replacement
•Control Module References for ECM replacement, programing and setup
