Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 270 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.8L)DIAGNOSTIC TESTSDTC P0420: CATALYST SYSTEM LOW EFFICIENCYCONDITIONS FOR RUNNING (LESABRE ONLY)
1999 GMC Forward Control 5.7 R, Automatic
Conditions For Running (LeSabre Only)
1999 GMC Forward Control 5.7 R, AutomaticSECTION Conditions For Running (LeSabre Only)
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2003 Pontiac Grand Prix, 2003 Pontiac Bonneville, 2003 Chevrolet Monte Carlo, 2003 Chevrolet Impala, and 2003 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
- Meet the conditions for engine warm up. Use the scan tool catalyst data list in order to verify the following:
- DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0206, P0300, P0336, P0341, P0401, P0403, P0404, P0405, P0410, P0412, P0418, P0440, P0442, P0443, P0446, P0449, P0502, P0503, P0506, P0507, P1133, P1134, P1336, P1351, P1352, P1361, P1374, P1441 are not set.
- The engine has been running more than 10 minutes.
- The Engine Coolant Temperature (ECT) is above 158°F (70°C) and below 255°F (124°C).
- The Barometric Pressure (BARO) is above 75 kPa.
- The vehicle is in Closed Loop.
- The Intake Air Temperature (IAT) is above -4°F (-20°C) and less than 212°F (100°C).
- The battery voltage is above 10.7 volts.
- Warm up the catalyst.
- Fully open the hood.
- Place the transmission in Park (A/T) or Neutral (M/T).
- Set parking brake.
- Press and hold the service brake.
- Each time the engine is started, the diagnostic can run up to 18 times. After the 10 minute run time and before the diagnostic runs and the first time, the engine must run an additional 5 minutes between 1500-2500 RPM. For any additional Tests on the same key cycle, the engine speed must be between 1500-2500 RPM for 1 minute.
- In order to activate the diagnostic return the vehicle to idle and put the vehicle in drive, or depress the clutch for a manual transmission.
- Test the catalyst.
- Place the transmission in drive (automatic) or neutral (for manuals with clutch depress).
- VIN K California Emissions
Within 60 seconds the air fuel ratio will go rich below 14.1 for up to 6 seconds, then it may go lean above 15.3 for up to 8 seconds. - VIN K Federal Emissions
Within 60 seconds the air fuel ratio will go lean above 15.3 for up to 6 seconds, then may go rich below 14.1 for up to 7 seconds. - VIN 1
Within 60 seconds the air fuel ratio will go rich below 14.1 for up to 7 seconds, then it may go lean above 15.3 for up to 9 seconds. - Check if DTC P0420 has passed or failed this key cycle using the scan tool.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.