Download Free Volvo 850 Turbo Engine Troubleshooting With Codes.pdf If no faults were found while performing BASIC DIAGNOSTIC PROCEDURES, proceed with SELF-DIAGNOSTIC SYSTEM. If no fault codes or only pass codes are present, proceed to H - TESTS W/O CODES article for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.). All voltage tests should be performed using a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless specifically stated otherwise in testing procedures.

850 turbo uses a Motronic 4.3 control system, which has a single Electronic Control Module (ECM) to control fuel injection and ignition systems. In addition, Motronic 4.3 control system incorporates an on-board diagnostic function. Signals from various sensors are monitored continuously. If certain signals are lost or become faulty, ECM will adopt fixed substitute values (limp-home mode) to enable vehicle to be driven for certain failures. Substitute values can be adopted for Engine Coolant Temperature (ECT) sensor signal, Throttle Position (TP) sensor signal, Mass Airflow (MAF) sensor signal, and oxygen sensor (HO2S) signal. See Fig. 1.

ECM stores Diagnostic Trouble Codes (DTCs) differently in accordance with a predetermined pattern. An emission related fault must be present for a predetermined number of intervals before ECM permanently stores a DTC and illuminates Malfunction Indicator Light (MIL). If fault is cleared before a specified interval has passed, DTC is erased from memory. If a fault disappears for any reason after DTC has been recorded permanently by ECM, DTC information will be retained by ECM. Each time the fault occurs, first counter will advance one. Each trip completed without a reoccurrence, a second counter will count down from 3 to zero. When second counter has reached zero, MIL will extinguish (if illuminated) and a third counter will start counting down. The third counter counts number of warm-up cycles (starting at 40). A warm-up cycle is when engine temperature is less than 104 F (40 C) at engine start and exceeds 183 F (84 C). When third counter reaches zero, DTC is erased from ECM memory. If a fault reoccurs, the second and third counter revert to their original settings and the first counter advances one.