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The old "pay me now or pay me later" adage still holds true for auto service. Putting off regular maintenance or ignoring signs of mechanical trouble can turn an affordable shop bill into a budget-buster that makes you pay for your procrastination. A squealing belt or shaky steering wheel are signs of trouble you should get squared away pronto. Veteran auto technicians contributed to this list of symptoms to watch for.

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Heed these warning lights

The low tire pressure warning light indicates that a tire-pressure monitoring system sensor has detected a tire that is significantly underinflated. The sensors are located within the air valve on each wheel and will typically warn when air pressure drops 25 percent below the recommended level. The TPMS will issue an instant warning in the case of sudden loss of pressure, such as from a puncture. If the TPMS light illuminates while you're driving, pull over as soon as possible and inspect your tires. Continuing on an underinflated tire is unsafe and can ruin a tire that could otherwise be repaired. Repairing a nail hole might cost $20, while a new tire could be $200. Tires will always lose a little pressure over time, and if your tire pressure has not been maintained it will eventually drop low enough to trigger the TPMS warning, often on a cold morning. Use a pressure gauge to check each tire. If they are uniformly low, it's time to fill them to the pressure specifications posted on a sticker inside the driver's door. Driving on low tire pressure hurts fuel mileage, handling, accelerates tire wear and can even lead to tire failure. If only one tire is low, you may have a slow leak and should get that tire checked soon.

The check engine light (often in the shape of an engine) indicates that there's an issue with the vehicle's emissions control system. Two common problems are a failed oxygen sensor in the exhaust or an air leak in the fuel system, which can be as simple as a loose or missing fuel tank cap. The check engine light is easy to ignore because the car may seem to be functioning normally, but if left untended a minor issue with the emissions system can get much worse. A failed oxygen sensor can be replaced for about $200, but if it's ignored for long it can cause excessive fuel in the exhaust to ruin the catalyst, which can cost $1,000 or more to replace. A professional scan of the vehicle diagnostic computer will help a technician pinpoint the problem.

A hyperactive turn signal indicator is usually a sign that the signal bulb on the erratically flashing side of the car is burned out. On some vehicles the "blinker" won't blink if the light is out. A new bulb may cost about $4, and you can often change it yourself following instructions in the owners manual. The alternative could be a visit with the police and a $50 fine for an equipment violation.

The service engine light (often the shape of a wrench) indicates that it's time for an oil change or other routine service, including changing the air filter, rotating tires and changing the differential or transmission fluids. There may be a code that appears with this light, and you can check the owners manual to see what the code indicates. Ignoring this service — by putting off an oil change for thousands of miles — can lead to more expensive repairs down the road.

The low washer fluid warning means it's time to invest $2 in a jug of bug juice and the few minutes it will take to fill the underhood reservoir. Your owners manual will show you how. It's a safety issue, of course, if you can't see through a dirty windshield. Do not keep pushing the washer button if the reservoir is empty. The fluid often cools and lubricates the pump. You'll burn out the pump if it has run dry; expect a $100 bill to replace it.

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