Check engine light for volvo 850 wont go off wat else can be the problem if i got a full tune up and put in new spark plugs plus it trembles alot and it shakes when i start it up plus wen i put it on reverse it feels like a hard Thump wen i reverse it???? could sumbody help meeh out Please.

Hi, I recommend you access the OBD panel to extract the trouble codes. Then send us the trouble codes for further advice. On 850 models the DLC is located in the engine compartment in front of the module box, behind the passenger side headlamp.

1. Open diagnostic socket cover and install selector cable into socket No. 2 for fuel injection codes or socket No. 6 (except Motronic systems) for ignition codes.
2. Turn the ignition switch to the ON position.
3. Enter control system 1 by pressing the button once. Hold the button for at least 1 second, but not more than 3.
4. Watch the diode light and count the number of flashes in the 3 flash series indicating a fault code. The flash series are separated by 3 second intervals. Note fault codes.

1. If diode light does not flash when button is pressed, or no code is flashed there is a problem with the soft-diagnostic system, proceed as follows:
   1. Check ground connections on the intake manifold, and the ground connection for the Lambda-sond at the right front mudguard.
   2. Check the fuses for the pump relay and the primary pump. On 240 models, fuses are located inside the engine compartment on the left side wheel well housing. On 760/780 models, fuses are located in the center console, just below the radio. On 740/940 models, fuses are located behind the ashtray. Access can be gained by removing the ashtray, and pressing upward on the tab marked electrical fuses press. On 850 models, the fuses are located on the left side of the engine compartment behind the strut mount plate. The fuses on 960 models are located on the far left side of the dashboard. The driver's door must be open to gain access to the fuses.
   3. Remove glove compartment, and check control unit ground connections.
   4. Turn the ignition switch to the OFF position. Remove control unit connector and connector protective sleeve.
   5. Check diagnostic socket by connecting a voltmeter between ground and No. 4 connection on the control unit connector. Reading should be 12 volts. If no voltage is present, check lead between control unit connector and fuse No. 1 in the fuse/relay box.
   6. Turn the ignition to the ON position, and install selector cable into the No. 2 socket on the diagnostic socket. Connect a voltmeter between ground and No. 12 connection on the control unit connector. Reading should be 12 volts. Press the button on diagnostic socket and note reading. Reading on the voltmeter should be 0 volts. If no voltage at the control unit is present, take reading at the diagnostic socket connector. If reading remains at 12 volts when button is pressed, check diagnostic socket.
   7. Connect a voltmeter between ground and the red/black lead on the diagnostic socket connector. Reading should be 12 volts.
   8. Connect a suitable ohmmeter between ground and the brown/black lead in the diagnostic socket connector. Reading should be 0 ohms.
   9. Turn the ignition to the OFF position. Connect ohmmeter between diagnostic socket selector cable and the pin under selector button. The ohmmeter should read infinity. Press the button and note the reading. The reading should be 0 ohms.
   10. Connect a suitable diode/multimeter tester, or equivalent, between the diagnostic socket diode light and the selector cable. Connect red test pin from the tester to pin under diode light and black test pin from tester to selector cable. A reading on the tester indicates correct diode light function. With no reading on tester, replace diagnostic socket.
   11. Check the system relay/primary relay by connecting a voltmeter between ground and the No. 9 connection on the control unit connector, then connect a jumper wire between ground and No. 21 connection on the control unit connector. The relay should activate and the reading should be 12 volts.
   
   
2. Press the diagnostic socket button. Note any additional fault codes.

1. Press the diagnostic socket button for the third time to see if a third fault code is stored in the memory. If the diode light flashes the same code 1-1-1, there are no other codes in the memory.

Fig. Fig. 1: Push button once for mode 1
Push button twice for mode 2

Push button three times for mode 3

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To clear the codes:

1. Turn the ignition switch to the ON position.
2. Read fault codes.
3. Press diagnostic socket button 1 time and hold for approximately 5 seconds. Release button. After 3 seconds the diode light should light up. While the light is still lit, press the button again and hold for approximately 5 seconds. After releasing the button, the diode light should go off.
4. To ensure that the memory is erased, press the button 1 time, for 1 second but not more than 3 seconds. The diode light should flash code 1-1-1.
5. Start and run engine. If engine will not start, correct the problem before proceeding and start over with Step 1.
6. Check to see if new fault codes have been stored in the memory by pressing the diagnostic socket button 1 time, for 1 second but not more than 3 seconds.
7. If fault code 1-1-1 flashes, it indicates that there are no additional fault codes stored in its memory.