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NTC-10A: Checking Ho2S Switching

NOTE: 1/1 Heated Oxygen Sensor (HO2S) is located in upstream position. On models with 2 upstream HO2S, 1/1 HO2S is located in left upstream position. On AN body (2WD) with 5.9L engine and BE/BR bodies with 8.0L medium-duty engine, 1/2 Heated Oxygen Sensor (HO2S) is located in pre-catalyst position. On all other bodies, 1/2 HO2S is located in downstream position. On models equipped with 4 HO2S, 1/2 HO2S is located in left downstream position. 1/3 HO2S is used on BE/BR bodies with 8.0L medium-duty engine only and is located in post-catalyst position. 2/1 HO2S is located in right upstream position on models with 2 upstream HO2S.
NOTE: 1/2 and 1/3 HO2S switching should be slower than 1/1 or 2/1 HO2S switching.
  1. Start engine and allow it to idle. Allow engine to reach normal operating temperature. Using scan tool, read HO2S states. If HO2S states are switching between RICH and LEAN, system is functioning properly. Test is complete. If either HO2S is not switching between RICH and LEAN, go to next step.
  2. If either HO2S is locked on LEAN, go to next step. If neither HO2S is locked on LEAN, go to step 8).
  3. Ensure engine is still idling. Check engine, brake booster and vacuum hoses for leaks. Repair vacuum leaks as necessary. Perform TEST VER-2A. If no vacuum leaks exist, go to next step.
  4. Ensure engine is still idling. Using scan tool, read HO2S voltages. If voltage is less than .1 volt for any HO2S, go to next step. If voltage is more than l1 volt for all HO2S, go to step 7).
    NOTE: For circuit identification and wiring diagram, see appropriate wiring diagram in WIRING DIAGRAMS article.
  5. Turn ignition off. Disconnect suspected HO2S connector. Disconnect PCM connectors. PCM is located in engine compartment. See PCM LOCATION table under SYSTEM DIAGNOSTICS. Clean and/or repair connectors as necessary. Using an ohmmeter, check resistance between ground and suspected HO2S connector, signal circuit. If resistance is less than 5 ohms, repair signal circuit for short to ground. Perform TEST VER-2A. If resistance is 5 ohms or more, go to next step.
  6. At this time, suspected HO2S is assumed to be defective. Replace HO2S. Perform TEST VER-2A.
    NOTE: 1/2 and 1/3 HO2S switching should be slower than 1/1 or 2/1 HO2S switching.
  7. Turn ignition on, with engine off. Using scan tool, reset adaptive fuel memory. Start engine and allow it to idle for at least 3 minutes. Ensure engine is at normal operating temperature. Using scan tool, read HO2S states. If 1/2 and 1/3 HO2S are switching states slower than 1/1 or 2/1 HO2S, test is complete. If HO2S switching is not as specified, perform NTC-13A: CHECKING ENGINE MECHANICAL SYSTEMS.
    WARNING: High fuel pressure may be present in fuel lines. Open fuel system with caution. See FUEL PRESSURE RELEASE.
  8. Turn engine off. Release fuel pressure. Connect fuel pressure gauge to fuel rail. Turn ignition on, with engine off. Using scan tool, actuate fuel system test. Allow fuel pressure gauge to stabilize. Stop fuel system actuation test. Monitor fuel pressure gauge for one minute. If fuel pressure gauge reading drops more than 10 psi (.7 kg/cm2 ), replace leaking fuel pump, fuel lines or injector(s) as necessary. Perform TEST VER-2A. If fuel pressure does not drop as specified, go to next step.
  9. Inspect air cleaner and inlet ducts for restrictions. Repair air cleaner and inlet ducts as necessary. Perform TEST VER-2A. If no restrictions exist, perform NTC-13A: CHECKING ENGINE MECHANICAL SYSTEMS.
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