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Home >> Volvo >> 1995 >> 850 GLT, 4D Sedan, Standard >> Repair and Diagnosis >> External Pages >> Different car >> Section 2 (Engine Controls - Tests W/Codes - Non-Turbo (A/T) - MOTRONIC 4.4) >> Circuit Tests >> DTC 521 - Front Ho2S Preheating Or DTC 522 - Rear Ho2S Preheating

DTC 521 - Front Ho2S Preheating Or DTC 522 - Rear Ho2S Preheating

WARNING: This page is about a different car, the 1996 Volvo 850. However, it is still accessible from the selected car via links, so may be relevant.
  1. Check status message using Volvo Scan Tool (998-8686 ). If PERMANENT FAULT, SIGNAL TOO HIGH is displayed, go to next step. If PERMANENT FAULT, SIGNAL TOO LOW is displayed, go to step 3). If INTERMITTENT FAULT, SIGNAL TOO HIGH is displayed, go to step 8). If INTERMITTENT FAULT, SIGNAL TOO LOW is displayed, go to step 9).
  2. If PERMANENT FAULT, SIGNAL TOO HIGH is displayed, turn ignition off. Disconnect front HO2S Black connector and/or rear HO2S Gray connector. Turn ignition on. Connect voltmeter between HO2S connector terminal No. 2 and ground. If voltmeter reads about zero volts, retest using new HO2S. If voltmeter does not read about zero volts, check wiring between HO2S connector terminal No. 2 and ECM connector terminal A14 (DTC 521) or terminal A29 (DTC 522) for signs of a short circuit to voltage.
  3. If PERMANENT FAULT, SIGNAL TOO LOW is displayed, turn ignition off. Disconnect front HO2S Black connector and/or rear HO2S Gray connector. Turn ignition on. Connect voltmeter between HO2S connector terminal No. 1 and ground. If voltmeter reads battery voltage, go to next step. If voltmeter does not read battery voltage, check wiring between HO2S connector terminal No. 1 and main relay connector terminal No. 3 for an open circuit.
  4. Turn ignition on. Disconnect HO2S connector. Connect voltmeter between HO2S connector terminal No. 2 and ground. If voltmeter reads about zero volts, go to next step. If voltmeter does not read about zero volts, check wiring between HO2S connector terminal No. 2 and ECM connector terminal A14 (DTC 521) or terminal A29 (DTC 522) for a short circuit to voltage.
  5. Turn ignition off. Wait about 2 minutes. Disconnect HO2S connector. Disconnect ECM. Connect ohmmeter between HO2S connector terminal No. 2 and ground. If ohmmeter reads infinite resistance, go to next step. If ohmmeter does not read infinite resistance, check wiring between HO2S connector terminal No. 2 and ECM connector terminal A14 (DTC 521) or terminal A29 (DTC 522) for a short circuit to ground.
  6. Turn ignition off. Disconnect HO2S and ECM connectors. Connect Breakout Box (981-3190) to ECM connector. Check all grounds. See SYSTEM/COMPONENT TESTS article. Connect an ohmmeter between HO2S connector terminal No. 2 and breakout box pin No. 14 (DTC 521) or pin No. 29 (DTC 522). If ohmmeter reads about zero ohms, go to next step. If ohmmeter does not read about zero ohms, check wiring between HO2S connector terminal No. 2 and ECM connector terminal A14 (DTC 521) or terminal A29 (DTC 522) for an open circuit.
  7. Turn ignition off. Reconnect HO2S connector. Disconnect ECM connector. Connect ohmmeter between breakout box pins No. 27 and No. 14 (DTC 521) or No. 29 (DTC 522). If ohmmeter reads 1.5-13 ohms, DTC is caused by poor contact in ECM and/or HO2S connector. If ohmmeter does not read 1.5-13 ohms, retest using new HO2S.
  8. If INTERMITTENT FAULT, SIGNAL TOO HIGH is displayed, check wiring between HO2S connector terminal No. 2 and ECM connector terminal A14 (DTC 521) or terminal A29 (DTC 522) for an intermittent short circuit to voltage.
  9. If INTERMITTENT FAULT, SIGNAL TOO LOW is displayed, check wiring between HO2S connector terminal No. 1 and main relay connector terminal No. 3 for an intermittent open circuit. Check wiring between HO2S connector terminal No. 2 and ECM connector terminal A14 (DTC 521) or terminal A29 (DTC 522) for an intermittent open circuit or short circuit to ground. Also check HO2S and ECM connectors for loose connection, contact resistance, and oxidation.
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