Q: I have a 2000 Lexus 300 ES which I recently put in the shop to have the timing belt and water pump replaced. The car runs great, but the horn on my security system is no longer audible when I lock and unlock the car. It locks and unlocks properly, but there is just no beeping sound. Before I put the car in the shop it worked just fine. My mechanic checked the relay, but can't figure it out. I miss the convenience of hearing the car lock and unlock, and hope you can help me out.

A: Best I can tell by researching this issue online, there is a volume control for the door lock/unlock horn mounted under the dash near the center console. Perhaps this got "unadjusted" during the vehicle's service.

Since it's very likely the technician disconnected the battery while working under the hood, the keyless entry system may need reprogramming. Apparently Toyota/Lexus service tools can provide customer customization, including turning on or off the remote door lock horn. Since your keyless remote transmitter still operates the door locks properly, this may be the answer.

And following the KISS principle of keeping it simple, check the fuse for this horn in the engine compartment junction block. This fuse provides power to the theft-deterrent electronic control unit (ECU).

Q: Two years ago I bought a 2003 Jeep Liberty Limited with automatic transmission that had 125,000 miles at the time of purchase and now has 141,000 miles. Recently I received advice to not change the transmission fluid and filter in the Jeep. I have always done this service with other vehicles at the 25,000-mile mark or after two years. I have checked the levels and fluid color and they appear OK. Why would the mechanic advise me not to do the transmission maintenance? He said that if I did the fluid change, it would cause issues with the transmission. Is he right, or is there another solution?

A: In today's world of sealed, no dipstick, virtually maintenance-free automatic transmissions, the transmission fluid and filter are easily serviceable in your Jeep. Not only is there a dipstick under the hood to allow checks of the fluid level and condition, but the transmission oil pan is removable to replace the fluid and filter.

The maintenance recommendations for your vehicle include changing the transmission fluid and filter at 100,000-mile intervals under "normal" conditions, but more frequent changes at 30,000-mile intervals under "severe" operating conditions.

I suspect your mechanic is assuming that the transmission fluid has never been changed in your vehicle. If that's true, oxidation of the original automatic transmission fluid (ATF) after nearly 150,000 miles could lead to sludge and varnish deposits that, if loosened or dissolved by the fresh ATF, could conceivably restrict hydraulic passages or components and cause potential problems or failure.

So, what to do? Can you research the vehicle's service records through a Jeep dealership? You might be able to determine whether or not the transmission has ever been serviced. If it has not and you plan to keep the vehicle, you have one of two options: 1) add half a can of SeaFoam Trans Tune to the transmission, drive the vehicle for a week or so and then have a complete flush, refill and new filter to clean, flush and totally exchange all the old fluid for new, or 2) do nothing as your technician suggested and hope for the best.

Obviously, if the transmission has been serviced in the past, continue to service it according to the factory maintenance schedule.

Personally, I'd go for the complete flush, fill and filter.

Q: I have a 1998 Ford Ranger with 135,000 miles that needs a fuel pressure regulator. I have been told that this part is obsolete and not available in this area. Can you tell me where and how to find it?

A: I found quality aftermarket fuel pressure regulators for the 3-liter and 4-liter V-6 from $50 to $100 online.