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 MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 113 (ENGINE CONTROLS SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0420: TWC SYSTEM - LOW EFFICIENCYCONDITIONS FOR RUNNING DTC
2003 Chevrolet TrailBlazer 4.2 S, 4WD
Conditions For Running DTC
2003 Chevrolet TrailBlazer 4.2 S, 4WDSECTION Conditions For Running DTC
WARNING: This page is about a different car, the 2002 Pontiac Aztek and 2002 Buick Rendezvous. 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 conditions:
- DTCs P0030, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0171, P0172, P0201-P0206, P0300, P0336, P0341, P0404, P0405, P0410, P0440, P0442, P0443, P0502, P0503, P0506, P0507, P1133, P1134, P1351, P1352, P1361, P1362, or P1441 are not set.
- The engine has been running longer than 10 minutes.
- The Engine Coolant Temperature (ECT) is between 169-255°F (76-124°C).
- The Barometric (BARO) pressure is above 75 kPa.
- The vehicle is in Closed Loop.
- The Intake Air Temperature (IAT) is between -4-212°F (-20-100°C).
- The battery voltage above 10.7 volts.
- Warm up the catalyst.
- Fully open the hood.
- The transmission is in Park, A/T, or Neutral, M/T.
- Set the 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 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 must be between 1500-2500 RPM for 1 minute.
- In order to activate the diagnostic, return the engine to idle and place the vehicle in drive, or depress the clutch pedal for a manual transmission.
- Test the catalyst.
- The transaxle is in Drive (A/T), or Neutral (M/T) with the clutch pedal depressed.
- Within 60 seconds the air/fuel ratio will go lean, below 14.1, for up to 7 seconds. The air/fuel ratio may then go rich, above 15.3, for up to 7 seconds.
- Using the scan tool, check and see if DTC P0420 has passed or failed this key cycle.
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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.