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Most likely just a bad knock sensor. You can try clearing the code and see if it comes back again later. If it comes back I would try replacing the rear knock sensor. Here's some troubleshooting from the shop manual, but in the end they recommend replacing the knock sensor anyway. You didn't recently clean the engine with a bunch of water did you?
Mike
'95 850T
Code 4-3-3 (Rear Knock Sensor Signal)
1) Check all wiring and connectors for poor contact.
2) Gain access to knock sensor(s) by removing upper charge air cooler
pipe and engine cooling fan. To gain access to rear knock sensor also
remove intake manifold support bracket.
3) Disconnect knock sensor. Connect an ohmmeter between knock sensor
connector terminal No. 2 and ground. If ohmmeter reads about
zero ohms, go to step 5).
If ohmmeter does not read about zero ohms, go to next step.
4) Ensure knock sensor is disconnected.
Ensure ECM connector is making good electrical contact
and is connected securely to ECM. Connect an ohmmeter between knock
sensor connector terminal No. 2 and ground. If ohmmeter reads about
zero ohms, code was caused by poor contact in ECM connector. If
ohmmeter does not read about zero ohms, check wiring between knock
sensor connector terminal No. 2 and ECM connector terminal A17 for an
open circuit.
5) Ensure knock sensor is disconnected.
Connect an ohmmeter between knock sensor connector
terminal No. 1 and ground. If ohmmeter reads about zero ohms, check
wiring between knock sensor connector terminal No. 1 and ECM connector
terminal A30 (4-3-3) for short to ground.
If ohmmeter reads about one megohm, go to step 7). If ohmmeter reads
infinite resistance, go to next step.
6) Ensure knock sensor is disconnected.
Ensure ECM connector terminals are free of resistance
and oxidation. Reconnect ECM. Connect an ohmmeter between knock sensor
connector terminal No. 1 and ground. If ohmmeter reads about one
megohm, code is caused by poor contact in ECM’s connector. If ohmmeter
does not read about one megohm, check wiring between knock sensor
connector terminal A30 (Code 4-3-3) for an open circuit.
7) Reconnect knock sensor(s). Start and warm up engine.
Increase engine speed to slightly greater than 3500 RPM. Use Volvo
Scan Tool (998-8686) to check knock sensor value. If knock sensor
value is 2-6 at speeds greater than 3500 RPM, source of code is poor
contact at knock sensor connector. If knock sensor value is not 2-6 at
speeds greater than 3500 RPM, test system using a new knock sensor.
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