2015 Kia Rio Automatic – Fixed Transmission, But Issue Came Back (I Know the Cause!)

So, I bought a 2015 Kia Rio (automatic) in December 2023, and at first, everything seemed fine. But by September 2024, I noticed a jerking issue when shifting from Gear 1 to Gear 2. Since this was my first automatic car, I thought it was normal—after all, automatics are just manuals with robots shifting for you, right?

Over time, the jerking got worse, and eventually, I had full-blown shift shock where the car refused to go beyond Gear 3. Reverse was even worse—it felt like the car was physically fighting the shift.

I took it to several mechanics, and finally, one of them pinpointed the issue: The ECU was completely responsible for everything. Apparently, the transmission itself was fine, but the ECU had developed some kind of electrical fault, which caused bad gear changes, jerking, and even shift failure. So, I did what any reasonable person would do—I replaced both the ECU AND the entire transmission because, you know, might as well cover all bases.

And guess what? It worked! Well… for about a week.

Yesterday, the same jerking issue came back, specifically from Gear 1 to Gear 2. But now, I’m certain that it cannot be the transmission or ECU because they are both brand new. That leaves only one possible explanation:

💡 I think my car is misfiring during gear changes because the spark plugs or fuel injectors are not synchronised with the automatic transmission shifts. Maybe the engine is hesitating at the exact moment it needs to upshift, making it feel like a gear issue when in reality, it's just a fuel delivery or ignition problem?

I’m taking the car back to the mechanic, but I just wanted to confirm—do you guys agree that misfires during shifts can cause this issue, and should I look at the ignition system next? Because at this point, there’s no way it’s the transmission again.