Q: The battery keeps going dead in my 2002 Jeep Grand Cherokee Limited (4.7-liter V-8 engine). The battery and alternator have both been replaced to no avail. Do you have any ideas as to what might be causing this to happen? -- A.C., Palos Hills

A: A.C., a technician needs to determine if the dead battery is being caused by a problem with the charging system or the result of an excessive current drain when the engine is turned off. To test for excessive current drain, everything needs to be turned off and the battery negative cable needs to disconnected from the negative battery post. Then, an ammeter should be connected across the disconnected negative cable and the battery’s negative post. There should be less than a 50-milliamp draw with everything turned off. If the current drain is higher than 50-milliamps, a technician will need to identify which circuit is staying on by disconnecting fuses and circuit breakers one at a time.

If the current drain is lower than 50-milliamps, the technician should focus on the charging system. Even though the alternator has been replaced, the wiring to the alternator should be inspected for possible problems. Also, the circuit that controls the alternator’s output resides in the engine’s powertrain control module (PCM). A diagnostic scan tool might be needed to test the charging system on your Jeep. Take your Jeep to a shop that specializes in electrical repairs and employs ASE-certified technicians.

Q: The “check engine” light came on in my 2000 Saturn SL2. I took it to the local auto parts store and they scanned it for me. There was only one code, a P0133, “oxygen sensor slow response.” The guy who scanned it said that I probably need a new oxygen sensor. I remember reading in previous columns that the code doesn’t always pinpoint a failed part but indicates an affected circuit. I don’t have a lot of money and can barely afford the new oxygen sensor. If I take it in for a diagnostic check at a repair shop, I know I won’t be able to afford the new oxygen sensor, too. The sensor is easy to get to and I can change it myself if you think that will fix the problem. What do you think? -- R.E., Calumet City

A: R.E., normally I would say take it in and have a technician make sure that the wiring for the oxygen sensor is OK. But a code P0133 is most typically caused by a degraded or failed oxygen sensor. If you feel confident in your skills, go ahead and change the sensor yourself.

You should know, though, that the “check engine” light won’t go off after you change the sensor. It should go off on its own after operating the vehicle for several days, but if it doesn’t, go back to the auto parts store and see if they can clear the code for you with their scanner.

If you need to go for an emissions test soon, drive your car for several days after the code has been cleared. Then, go back to the auto parts store to see if all your car’s OBDII monitors are “ready.” Don’t waste your time going for an emissions test until all your car’s OBDII monitors are “ready” or your car will fail.

AutoAdviser: Ira Siegel is an Automotive Instructor and ASE Certified Master Auto Technician. If you have auto-related questions for Ira Siegel you can call and leave your questions at the Auto Adviser Hotline at 708-633-6839 or e-mail a question to Autoadvisr@aol.com.