Replaced engine twice, keeps giving misfire codes?

2011 TOYOTA PRIUS
230,000 MILES • 1.8L • AUTOMATIC • HYBRID
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2011 prius. replaced engine twice, keeps giving misfire codes and catalyctic converter over heats. replaced cat. no change in misfire code. It started with cylinder 1 and then 4. Keeps changing
Oct 1, 2026 at 5:55 PM
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JACOBANDNICKOLAS
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  • 110,244 POSTS
Hi:

If you have a scan tool that has real time or live data, connect it to the OBD port. Start the engine and allow it to reach operating temperature. Once it does, go to live data and check the short term fuel trims and let me know what they are.

They should be between -10 and +10. 0 is perfect.

Here is a link you may find helpful.

https://2carpros.com/articles/engine-misfires-or-runs-rough/

Let me know what you find.

Joe
Oct 1, 2026 at 8:40 PM
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KRAKENAUTO
  • ✓Experienced Mechanic
  • 97 POSTS
Is the engine you replaced with used? Is the 12v battery also fully charged?

Gen 3 Prius engines are prone to head gasket failures, which allow coolant to seep into the combustion chambers. This causes random misfires (often on cylinder 1 or 4 due to coolant flow design) and destroys catalytic converters.

Check your coolant reservoir level for loss and inspect the spark plugs for a clean, steam-cleaned appearance (white/shiny electrode) and let us know those results.
Oct 4, 2026 at 7:03 AM
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