V70 XC AWD L5-2.4L Turbo VIN 56 B5254T (1998)
Twin heated oxygen sensor (HO2S) control starts and remains active when:
^ The front heated oxygen sensor (HO2S) has started control
^ The rear heated oxygen sensor (HO2S) has reached operating temperature
^ Catalytic converter temperature exceeds +300°C (calculated by the engine control module (ECM))
^ Engine speed is between 1,200-3,400 rpm
^ Engine load is 1.3-5.0 ms (at 1,000 rpm), 1.3-4.0 ms (at 2000 rpm), 1.3-3.0 ms (at 3000 rpm).
Twin heated oxygen sensor (HO2S) control is suspended during fuel shut-off, if the engine control module (ECM) detects misfiring (diagnostic
trouble codes (DTCs) EFI-45X, 543, 545, 55X) or if the engine control module (ECM) detects a fault in:
^ EVAP system (diagnostic trouble code (DTC) EFI-315)
^ Three-way catalytic converter (TWC) (diagnostic trouble code (DTC) EFI-443)
^ Heated oxygen sensor (HO2S) heating (diagnostic trouble code (DTC) EFI-521, or EFI-522)
^ Mass air flow (MAF) sensor (diagnostic trouble code (DTC) EFI-121).
A defective sensor may also have the correct mean signal voltage. For this reason the front sensor period is also monitored. The engine control module
(ECM) monitors how long it takes for the front heated oxygen sensor (HO2S) to switch from rich to lean to rich again. This is carried out at partial
load when the difference between a good and bad sensor isgreatest. When the period becomes too long for a number of diagnostic checks, diagnostic
trouble code (DTC) EFI-435 front heated oxygen sensor (HO2S) slow response, is posted.
