06 cobalt

The 06 Cobalt, like most GM cars, uses two shift solenoids on the outside and inside the valve body. The way they line up electrically causes your car to shift.

Since you have lost fifth gear — the Cobalt of that era used a five-speed automatic — it is most likely solenoid two that has failed. At that it is likely only one side of the solenoid that has failed as you have gears 1-4.

Fifth gear is actually an overdrive type of gear that lengthens the gearing so your engine turns more slowly for the speed. It is one of the ways your Cobalt gets the reasonable mileage that made it popular.

I would suggest a couple of things to make sure your 12-year-old car remains in good shape — at least for transmission health. The first is taking it to a dealer (Chevy) and having one of the techs there run a full OBD-II diagnostic. This should reveal if there are any other problem areas that are going (Today’s OBD-II diagnostic is pretty comprehensive and while the scanner generally points at overall areas where problems may occur, the software has been refined to the point that you should find out where other problems may lie. It will also show you which solenoid pack has failed.).

Second, Id have the transmission drained and refilled to ensure it operates properly. Some shops use a high-pressure style transmission drain that I don’t advocate. The reason is that the high pressure jet may dislodge crusted deposits that could remain within the transmission, causing further damage later on. The proper way to drain the transmission is the remove the drain screw/plug and draining it into an environmentally friendly container (one made for this purpose). This will leave about a quart or so within the transmission pan. To remove this last quart there are 14 or so nuts that have to be backed out so the cover comes off. Be sure you have a bucket underneath to catch the fluid. Also, take a look at the drain plug as it is somewhat magnetic, to see if there are any metal shavings or shards that could indicate internal damage. with everything drained, replace the cover, being sure that you are using the right gasketing material (some use a fiber-style gasket, while others use a variant on RTV (room-temperature vulcanizing agent)), and replacing the nuts. Don’t overtorque them because you risk warping the cover. Hand-tight and a quarter-turn or so is all you need. While the transmission is open, replace the filter that you will also find in the sump. Refill with the recommended transmission fluid (it is in the owner’s manual), using the right number of quarts. You refill through the filler neck.

Let me know what happens.