Who did the timing belt work? Yes if it's off a little bit it can trigger the knock sensor. You can run through these simple tests to eliminate the knock sensor if you want to but the recent timing belt work is very coincidental.
If you have someone that has a Volvo VST or Tech2 scantool they can take the readings straight off the knock sensors and see if they really might be the problem. But you can do most of the test with a multimeter.

Mike
'95 850T




Code 1-4-3 (Front Knock Sensor Signal) &
Code 4-3-3 (Rear Knock Sensor Signal)

1) If fault is intermittent, check camshaft setting. See the
2.3L article in the ENGINES section. If camshaft setting is okay,
check all wiring and connectors for poor contact. If fault is
permanent, go to next step.

2) Turn ignition off. Check camshaft setting in relation to
crankshaft. Repair as necessary. If camshaft setting is okay, 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) Ensure ignition is still off. Wait 2 minutes. 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 ignition is still off and 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 ignition is still off and 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 A2 (Code 1-4-3) or 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 ignition is still off and 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 No. 1 and ECM terminal A2 (Code 1-4-3) or 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.