Backfire when started

Tiny
GIJI79
  • MEMBER
  • 2014 CHEVROLET SONIC
  • 1.8L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 125,000 MILES
I had the codes read when my check engine light came on, the dealership said bad O2 sensor. So I changed them both. Within two days the light was back on and I took it to O'reillys and the same codes came up. I replaced both sensors again. This morning when I started the car it backfired out of the tail pipe and the light came on again. Could it be the catalytic converter that needs to be changed?
Monday, April 16th, 2018 AT 7:03 AM

3 Replies

Tiny
STEVE W.
  • MECHANIC
  • 15,237 POSTS
What are the exact codes? Most O2 sensors get changed out when there is nothing wrong with them. 90% of the time the issue is wiring or a fault farther upstream like an exhaust leak that lets in air or a vacuum leak causing a lean mix.
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Tuesday, April 17th, 2018 AT 8:30 PM
Tiny
GIJI79
  • MEMBER
  • 2 POSTS
P2270
P0171
P0442
My apologies for the delayed reply.
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Sunday, April 29th, 2018 AT 10:32 AM
Tiny
STEVE W.
  • MECHANIC
  • 15,237 POSTS
P2270 - O2 Sensor Signal Biased/Stuck Lean - (Bank 1 Sensor 2)
Code criteria - The ECM detects that the heated oxygen sensor (HO2S) 2 sensor cannot achieve voltage less than 775 mV. AND
The ECM detects that the accumulated mass air flow monitored during the Stuck Lean Voltage Test is greater than 60 g.

90% of the time this is a wire from the sensor to the ECM shorted to voltage or the ground to the sensor is bad.

P0171 - OBD-II Trouble Code: System Too Lean (Bank 1)
With the sensor stuck lean the system will constantly try to add fuel. That will also cause the backfire as the constant added fuel will collect in the converter and the fumes collect and ignite at the first spark. Check for a large vacuum leak or low fuel pressure.

P0442 - Evaporative Emission Control System Leak Detected (small leak)
This can be just about anything in the EVAP system, a smoke test should show where the leak is. This leak is about this size -- O Not something that you will see easily.
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Monday, April 30th, 2018 AT 8:30 AM

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