1- Crumbs or part wearout may be keeping the plastic latch from engaging. If you have the toaster apart, you can compare the arm latching action of the good side with the bad side--maybe there is an obvious difference in the travel of the arm that keep it from engaging.---leaving the travel short even 1/8" can keep the latch from engaging.
2- If there is nothing obviously mechanically wrong, then it may be a control board malfunction. Make sure the 3-pin control board connector is plugged into the transformer/solenoid board. Since you are getting lights, this should be OK. You have at least one good board (the side that works) so you could plug that board into the other side to see if the arm stays down. For fun, you can plug the bad board into the other side to see if the other side now malfunctions. If the problem follows the board, then you have your answer.
If you know someone that understands electronics, they may be able to repair the board--have them look at electrolytic capacitor C1 and make sure it is good. If that part is bad, it can be replaced for about $1 by going to Fry's and purchasing a replacement capacitor. After a couple of years of use, a defective C1 may "dry out" and cause control board problems.
- I repaired the same symtom that happened to one of the sides of my CPT-180 toaster. The magnets wouldn't engage due to a bad C1 capacitor on the circuit board that controlled that side of the toaster--the capacitor was bulged out on the bottom and turned out to be bad. I've attached some photos showing C1 bulged out compared to C1 on the good board.
C1 is the main power supply capacitor and if it goes bad, the circuit board power supply won't function correctly. This can lead to a broad spectrum of problems like the lever not staying down due to the magnets not engaging or the toasting control/timing circuits malfunctioning. If you replace the capacitor, it is best in most cases to replace with the same value as the orginal (220uf on my toaster)---you can usually get a replacement capacitor at Fry's Electronics. Replacing with a higher capacitance may be acceptable, but you should understand the impact to the rest of the circuit before you substitute a higher capacitance value. The voltage rating and temperature rating of the replacement capacitor should be the same or greater or the capacitor will fail quickly. Note the max temperature rating of capacitor C1 is +105C---toasters get hot (duh!) and the temperature around the circuit board of this model gets quite warm, so the temperature rating of the capacitor is critical. Finally, make sure the substitute capacitor fits when installed (requires soldering) and does not interfere with surrounding circuitry or other toaster mechanics when the circuit board is put back in place.
C1_location (next to screw)
Comparing_good_C1_with_bad_C1--note bulge on bottom of right C1
Bad_C1 showing capacitance, voltage and max temperature ratings
Jun 24, 2014