Has Volvo issued a service bulliten for oxygen sensors in the 5 cylinder V70 series? Both my 1998 and 2002 V70s have had weak oxygen sensors. My trustworthy private mechanic also drives volvos and recommends that I ignore the warning as long as the gas mileage does not decrease.
Oxygen sensors seldom go bad. When they do it’s usually because of some other influence on them. (lack of maintenance, misfires, rich running, oil consumption, coolant in the combustion chambers, etc.)
What do you mean “weak” oxygen sensors? Weak as in you need to replace them? Weak meaning your car didn’t pass smog test?
“Weak” as described by the error code readout. Volvo must have realized they had a problem, as they have designed a “replacement kit” that can be installed for several hundred dollars. The vehicles run without symptoms, except the “service required” message on the dashboard. The message is displayed sporadically.
A look at ALLDATA does not show any service bulletins in regards to lazy O2s. There is one related to rerouting the wire harness on the 98 but that’s it.
Your cars are, respectively, 14 and 10 years old so it’s at least possible to have lazy O2s based on age and mileage.
As far as I know, all Volvo O2s are manufactured by Bosch and Bosch has provided reliable O2s for decades to just about everyone under the sun.
You might consider posting the exact codes and maybe this can be sorted out. Knowing the mileage on the vehicles, if bought new or used, any history of overheating or oil consumption, etc, may also help.