2003 Chevy Prizm pinging, check engine light, varying fuel

P0171= system too lean, p0420= catalsyt efficiency below threshold. If you have a scanner that can read live sensor data, check the 02 sensores, the bank 1 sensor one should keep switching between high and low end of the scale, and the bank 1 sensor 2 should remain mostly in the middle, if this is happening, you converter is probably NO goood! Also check for vacuum leaks and fuel pump pressure for cause of the system lean code. It's dumping fuel to compensate for the PCM thinking it needs more gas!

I also recalled that Toyota Corrolla has a bulliten on the Car, I believe Prizm is Chevy's twin to Corrolla? If so try this out, may be covered by the dealer?
M.I.L. "ON" DTC P0420 TECHNICAL SERVICE BULLETIN Reference Number(s): EG028-04,  Date of Issue:  October 8, 2004 TOYOTA: 2003 - 2004 Corolla SECTION: ENGINE Superceded Bulletin(s): EG028-04,  Date of Issue:  July, 19 2004 Related Ref Number(s): EG028-04 ARTICLE BEGINNING INTRODUCTION NOTE: October 8, 2004: Part number for replacement ECM (PCM) in Fig. 3 has been updated; Reference to section in Technical Information System (TIS) has been updated in "ECM (PCM) Replacement Procedure. The previous TSB should be discarded. Some 2003 - 2004 model year Corolla vehicles equipped with a 1ZZ-FE engine may exhibit a M.I.L. "ON" condition with Diagnostic Trouble Code P0420 (Catalyst System Efficiency Below Threshold) stored in the Engine Control Module (ECM) (SAE term: Powertrain Control Module/PCM). Follow the repair procedure to reflash the ECM (PCM) and replace the catalytic converter assembly. NOTE: The repair procedure in this TSB is the same as TSB No. EG016-04 "Excessive Sulfur Dioxide Odor." This repair should only be performed once. APPLICABLE VEHICLES 2003 - 2004 model year Corolla vehicles equipped with the 1ZZ-FE engine produced BEFORE the Production Change Effective VINs shown below. NOTE: Some 2003 model year Corolla ATM vehicles with Delco ECM (P/N 89666-02150) are not reprogrammable. If the Diagnostic Tester displays an error message, confirm part number and then refer to the ECM (PCM) Replacement Procedure of this TSB. ECM (PCM) REPROGRAM PROCEDURE Check for the Authorized Modification Label affixed to the vehicle in the location shown in Fig. 3. If there is no label present, proceed to step 2. 1. Connect the Diagnostic Tester to the vehicle and select "CURRENT ECU CAL" from the "ECU REPROGRAM MAIN MENU." To determine the vehicle's current ECM calibration, see Fig. 2. 2. Fig. 2: ECU Reprogram Main Menu Proceed to Flash Reprogram the ECM (PCM) by following the procedures in TSB No. SS002-01 titled " ECU Flash Reprogramming Process, " for 2001 and later applicable models. 3. NOTE: Errors during the flash reprogramming process can PERMANENTLY DAMAGE the vehicle's ECM (PCM). In order to minimize this risk follow the steps below: Make sure that the current battery voltage is no less than 11.4 volts. " Install a battery charger and set on a low setting to assure that the battery voltage DOES NOT FALL BELOW 11.4 VOLTS during the reprogramming. " If battery's state of charge is in question, test with SST No. 00002-MP815-T, "Midtronics Battery Tester." " Confirm that the engine hood is up, parking brake is set and all accessories are OFF (i.E, audio system, A/C, interior lights, headlights, etc). " DO NOT perform the reprogramming process until the engine room temperature is 158 °F (70 °F) or less. " Confirm cable connections between the vehicle and the Diagnostic Tester are secured. " In case of Error Message "Cannot Go To Reprogram Mode, " refer to TSB No. SS004-01 "ECU Flash Reprogramming Error Message. " " Do not interrupt the vehicle during the reprogramming process, keep all doors closed. " Select the proper ECM (PCM) calibration file for the model year vehicle and reprogram the ECM (PCM). 4. Start the engine and warm up the vehicle to normal operating temperatures before test- driving. 5. Test drive vehicle to confirm proper operation and to readjust the ECM (PCM) "Learned Values." 6. Drive the vehicle using light to moderate acceleration in mostly stop and go type driving. " Allow the vehicle to shift fully through all gears and at various shifter positions. " Enter the following information on the Authorized Modification Label shown in Fig. 3 using a permanent marker. 7. ECM (PCM) part number. " New ECM (PCM) calibration number. " Your dealer code. " Reprogramming date. " This TSB number. " Fig. 3: Locating Authorized Modification Label ECM (PCM) REPLACEMENT PROCEDURE 2003 model year Corolla ATM vehicles with Delco ECM (PCM) P/N 89666-02150 are not reprogrammable. 1. For the location of the ECM (PCM) refer to the Technical Information System (TIS), 2003 or 2004 model year Corolla Repair Manual: Engine Control System: ECM (1ZZ-FE) (April, 2003): Replacement (Corolla). 2. CATALYTIC CONVERTER REPLACEMENT PROCEDURE Refer to TIS, 2003 or 2004 model year Corolla Repair Manual: Exhaust: Exhaust Pipe Assy: Removal and Installation and Disassembly and Reassembly (Corolla).