After last year's squeeze on credit and the use of credit cards, consumers started to cash a lot more checks.

And as more checks were used to pay for goods and services, more bounced back from the banks stamped: "Insufficient Funds."

To avoid getting burned by bad checks, more and more commercial businesses are joining one of the major check verification and guarantee networks.

Perhaps you've wondered what the store clerk does with your check and driver's license after a sale. Your license number, amount of the check and some coded numbers are murmured into the phone while you wait.

At the other end, an operator who works for TeleCheck, TeleCredit or some other check guarantee company, punches the coded numbers into a computer.

In a split second, the video screen lights up with a "Code One," which means there's no negative check-writing information on you, or a "Code Four," which means one or more of your previous checks has been recorded as a bouncer.

If you get the favorable code, there's no problem. You just pick up your parcel and go. But if you get the negative code, the clerk says your check can't be accepted because the verification company won't guarantee it.

You are supposed to be given a card with the phone number and address of the check verification company. When you call the company, your name, driver's license (and other identification) are punched into the computer.

Your record is revealed and the bad checks are noted (date, amount and place you cashed them). In some instances, there's an error. Perhaps you didn't keep close tabs on your checking account balance and a check was returned to a store. You were notified, and made good on the bad check with embarassed apologies.

But in the hustle of daily business, someone at the store forgot to phone the check verification company that your previously reported bad check had been paid up in full.

If this is the case, the check protection company makes a call to verify your explanation. Upon getting the word that your bill is paid up, the operator erases your negative record from the computer bank. You're clean.

On the other hand, if you have not covered a check that bounced, your negative record stays intact until you do. This can be tough. Everywhere you go where check verification and guarantee systems are in place, your checks won't be accepted.

And more and more merchants are signing up for the service because they want to limit their losses from bad checks. Members pay a two to three percent commission on the amount of each check cashed, which is just about what most Visa, Master Card and other credit card member outlets pay for their credit services.

The check verification industry is in the process of installing electronic terminals so member outlets can process checks much faster.

As a consumer, you can speed up the process by joining a check verification and guarantee organization. Some local banks have special cards that guarantee their customers' checks in the area.

And TeleCredit has a "Welcome Check" I.D. card which guarantees your checks within a national network of member merchants. It costs $6 a year.

For more information, you can usually find check verification companies listed under "Check Cashing Protection Systems" in the phone book yellow pages.

Q. I got a letter from my Cadillac dealer recommending that I install a "Water-In-Fuel Sensor Kit" in my car. The letter explained that water in diesel fuel can damage certain engine components. Is this device necessary? If so, why didn't they put it in the engine at the factory?

A. American diesel cars -- those manufactured before March 1980 -- are known to have had problems with water damage. When water gets into the fuel injector pump, it can really mess things up.

Replacing one of these pumps can cost as much as $300. You can get one of the "Water-In-Fuel Sensor Kits" that Cadillac mentioned for around $50.

Some dealers will charge for the installation (an additional $75) and some won't. Some dealers have set an installation deadline for free service. Check them out.

Most foreign-made diesels already have a water sensor system. But, you should always check with the dealer before you buy.