Hey Mike,
The DTC 435 means front O2 sensor slow responce. What it means is the front O2 sensor signal is slower than the ECU determines as correct. This can be caused by a few common things. First, it is important to understand how the O2's work. The front O2 sends info to the ECU on how the engine is performing. The rear O2 sends info on how the cat. conv. is functioning and moniters the front o2 performance. The rear o2 can trip a front o2 code but not vise-versa. The ECU believes the rear is always correct and this is where is gets tricky. Without a scan tool there is no way to tell which sensor is at fault. Most likely the front. It is also possible that the intake has a vacuum leak. Pay close attention to the vacuum elbows in and around the throttle body. They tend to gat soft and deteriate. You can go to the dealer and ask how much to diagnose using which o2(usually 1/2hrs). Going to a garage could be cheaper but if they do not have a Volvo scanner, they may get it wrong.

Good luck,
Gazzer