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Q: My Ion is misfiring or losing power intermittently. on 2003 Saturn Ion

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It loses power sometimes at highway speed - the RPMs reduce and and then power returns. It happens more often when trying to accelerate from a stop sign. It is intermittent. I have installed new spark plugs and the only engine code I get is P1133.
The P1133 is a slow reacting Oxygen Sensor, which may a result of the misfiring. If the engine is misfiring, are you getting misfire codes like PO300 or P0310 or P0302 etc. because, you should be.
It may feel like a misfire, but is really something else. Try disconnecting your Oxygen Sensor and test driving the car for about 15 minutes. You may be experiencing a lean misfire from a defective Oxygen Sensor,. If the symptom changes or goes away, bingo. If not, take it to a shop for a Pro Diagnostic tech to figure it out.
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