Drive it forever

Q I have been hearing and reading about bio diesel and how it can be a good substitute for regular diesel. However, this may not be the case. We own a 2006 Ford diesel truck and Ford Technical Service does not recommend blending regular diesel with more than 5 percent bio diesel, which is almost nothing. Why is this?

A Bio diesel has different seal-swell rates for rubber parts than mineral diesel. So there could be an issue with leaking seals after long-term use of the bio diesel. It also has a tendency to make contaminants in the fuel tank, tanker truck, and underground storage tank lift up and float around, clogging filters.

My guess: Ford is just being conservative because it does not want to pay any warranty claims caused by seal failures or clogged filters. Some other manufacturers permit a higher percentage of bio diesel, and most vehicles will run fine on as much as 20 percent. But wait, there is more! Emissions of NOX can be slightly elevated when diesels run on bio diesel, and Ford has to certify that these vehicles meet federal emissions standards, and that's a lengthy and expensive procedure.

I firmly believe that newer vehicles will be designed to run properly on bio diesel, but in the meantime, you stand the chance of voiding your warranty if you ignore Ford's recommendations.

Q We own a 2006 Toyota Sienna that I want to use for towing a light trailer. My problem is that even though the Sienna has a Toyota trailer hitch and the necessary wiring package, the trailer's taillights will not permit the Sienna's hazard flashers to work. Brake, turn signal, and running lights work perfectly. But I get one flash from the hazard lights, and then nothing. If I disconnect any one of the four taillights on the trailer, the remaining three flash perfectly. The problem is simple, the solution is not. It is clearly an overloaded flasher module, but Toyota does not offer a heavy-duty replacement. The dealer's brilliant solution was to have me remove two of the four bulbs on the trailer.

A Like many late-model cars, your Toyota uses an integrated flasher-turn-signal module, and there seem to be no after-market, heavy-duty upgrades for most of these. Any high school vocational-tech student should be able to rig up a simple pair of relays that would let you switch as many lights as necessary. Just put the relay coils in parallel with the stock taillight filaments, and use the relay's normally open contacts to turn the trailer lights on and off.

Or, if you would prefer, here's a simpler option: Swap out the incandescent lamps on the trailer for LED bulbs, which draw far less current.

Q I have a 1998 Dodge Grand Caravan with about 85,000 miles. The other day my wife came home from work and pulled into the garage and her headlights, parking lights, and taillights were on, even though she had not turned them on. She flipped the headlight switch back and forth several times but they didn't go off. While she was on the phone telling me about the problem, the headlights, parking lights, and taillights went off. When I got home about an hour later and came in through the garage, all of the lights were on. While I was lifting the hood, all of the lights began flashing very fast, like a strobe light, much faster than if your hazard switch was engaged. After I observed for a while, all of the lights would strobe for a short while and then stay on for a while, then maybe go off for a short while, then come back on, then maybe strobe some more. While driving the van to work I did find that as long as I had the headlight switch turned on, everything was fine, but if I left the switch in the off position, then they would alternate between strobe and on.

A It is either the headlamp switch or the Daytime Running Light module, if you have DRL. Get out your voltmeter or even just a 12-volt test light and start chasing it down.

Mike Allen is a senior editor at Popular Mechanics. Reader questions can be e-mailed to driveit@nytimes.com.