2001 Pontiac Sunfire P1336 Error Code: What Does It Mean and How to Fix It?

Hi everyone, I'm Kristin — a female car owner with no mechanical background, so please don't mock me for asking basic questions. I bought my 2001 Pontiac Sunfire from Carmax a few months ago. A few days after the purchase, we had a repair done where the vehicle's engine control module (ECM) was replaced because it failed. Shortly afterward, the check engine light came on. My instinct told me: 'If the computer was just replaced, why would the check engine light be on?' So I assumed the issue might be related to the ECM. When I took the car in for diagnosis, the mechanic said my front brakes were at 3mm thickness and needed replacement, along with spark plug service. They also recommended a throttle body and fuel injection cleaning due to a lean misfire error code — specifically P1336. I found it odd that the brake warning was emphasized so strongly by Carmax staff, which made me laugh and decide to decline all but the spark plug replacement. I took the car to a friend at Midas who performed a rust removal on the rotors and advised me to return in 8,000–10,000 miles with no charge. Now I'm confused: What does error code P1336 actually mean? Is it related to the brake system or something else? The car runs fine otherwise — no performance issues, just the persistent check engine light. I've tried disconnecting the battery, yelling at the car, and even kicking the tires — nothing worked. Can anyone explain what P1336 means in a 2001 Pontiac Sunfire? And how can I resolve this error code without costly repairs? Thanks for your help! — Kristin