Check Engine Light
 

My '18 BRZ with only 7200 miles failed the smog test. The check engine light came on and then off again during the period prior to the test. When the test was done, there was no light. Codes stored were P0700, U0073, and U0122. Codes were cleared by the dealer and smog certificate issued. The car was fine for 2 months. Yesterday, the car would not start and the check engine light came on again. After about an hour, the car started as if nothing was wrong. I did notice that the air conditioning temperature changed from the previous setting so something must have been reset. The faults indicated that the transmission module lost communication. The dealer checked the wiring/harness for damage, and found none. Anyone have ideas? Bad control module????

Have any mods? MT/AT?

Give all electrical connections a look, such as the battery terminals. I had wiring issue where battery terminal nut connects to alternator current sensor and it was loose enough to complete circuit when not moving but not while driving and it was causing my ac to not kick in full "cold" just blew breezy.

Does your clock ever reset or does it hold time?

Does the brake pedal feel still "normal" or has it changed? IMHO, if its harder to press and with those codes... think it'd lead me to check abs module/pump.

Still new to troubleshooting, so just my 2c.

No new mods. In fact, other than accessories, no mods at all. Brakes are fine. Clock does not reset. I am getting a new battery since the one I have came with the car. But the battery did easily start the car an hour after I had this issue. I don't drive the car a lot and it is always garaged. After I install the new battery, we'll see if the problem reappears.

Check the connections from the ECU to the TCM module. Make sure you don't have any corrosion or anything.

Also check your battery.

The connections are fine. That said, I did change the battery at the suggestion of the dealer. It had the original battery in it. In addition, I changed the battery in the FOB which was also old. I hope that this solves the problem and it was that simple. Thanks for the suggestion.