Q. I recently replaced the oxygen sensor on my 2001 Corolla due to the check engine light. How long will the car need to be driven once I disconnect the battery and reset the check engine light so I can have the car inspected?

A. The computer system in the car continually monitors individual emission components. When the battery is disconnected these monitors revert to “not ready” status. The readiness monitors will remain until the car has been driven long enough for the computer to evaluate the various system and sensors. The amount of time can vary based on the vehicle. In some cases it can take up to 100 miles for all of the computer monitors to reset completely.

Q. We purchased two Honda 2002 CRVs about six months apart. At 25,000 miles the brakes started to squeal in one of the vehicles, and we replaced the rear brakes. Shortly after, the same problem presented itself in the second CRV. Now at about 100,000 miles, the check engine light started coming on, and we needed to replace the oxygen sensor at a cost of about $450. Like clockwork, the same problem is showing up in the second CRV. My question is, what are the ramifications if I fail to replace the oxygen sensor now?

A. When the check engine light is on, it indicates the engine emissions system is malfunctioning and possibly polluting the air and wasting fuel. If it were me, I would have the vehicle tested and the fault repaired as soon as possible. If it is a faulty oxygen sensor, over time it could damage the catalytic converter.

Q. I have a hard-starting 1997 Chevy S-10 pickup. I have replaced the spark plugs, plug wires and fuel filter. Sometimes it runs fine, but other times it hesitates. Do you have any suggestions?

A. Although you have done a fair amount of maintenance, one area you may have overlooked is fuel pressure and volume. When reading the fuel pressure using a gauge, you should see 60-66 pounds of pressure with the “key-on-engine-off.” If the pressure is less, it can result in hard starting and hesitation.

Q. The rear tires of my 1997 Ford Aerostar are making noise like the front tires are dragging them. A mechanic told me I need to change the differential. Do I need to change the whole differential or just the differential bearings?

A. When you are looking at a 13 year-old vehicle, it may actually be more economical to replace the entire differential with a used unit from a salvage yard rather than rebuilding the one in the truck. At this point, I would get a second opinion regarding the necessary repairs and make the decision based on cost.

Q. I have a 2004 Infinity G35. I hear squeaks out of the area of the tires or shocks when I drive over an uneven road. Just passive motion up and down causes the issue. It sounds like something needs to be lubricated around that area. Do you have any suggestions?

A. The problem is most likely a suspension bushing. Because the noise is easily duplicated, a technician should be able to pinpoint the source on a “drive-on” style lift. The most common problem with this model Infiniti is a lower control arm bushing. The bushing is not serviceable and the control arm assembly will need to be replaced.