1996 Pontiac Grand Am P0304 P0420

GMC
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JOHN SAVINO
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  • 1 POST
i have a 1996 grand am with 40000 miles the check engine light came on with fault code p0420/p0304. can the coil housing be causing this.after I reset code it stays off till the weather gets wet.
Jan 22, 2006 at 12:14 PM
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MVANNESSJR
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Easiest thing to do with p304 is to check your plugs (specifially #4) and make sure it is gapped properly and it is not oil fouled. I've had this problem before and in my case I found that the plug actually backed itself out of the block and needed to be tightened a bit back down. Be careful not to overtighten the plug as you can break it off into the block.

As for your other trouble code, this can be tripped due to a retarded spark.

I would investigate your plugs before you go looking for the coil pack.

Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). The voltage should be low and steady (~200mV, depends on the vehicle). If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

If your plugs are fine and the sensor checks out then you may need a new coil pack and I am sure you could find one in good shape at the local junk yard for next to nothing.
Jan 22, 2006 at 11:46 PM
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Suggested Repair

Check spark plug 4 for proper gap, oil fouling, and tightness without overtightening. Then measure Bank 1 rear oxygen-sensor voltage; it should stay low and steady around 200 mV, while varying voltage points to the catalytic converter. Test the other O2 sensors as well. If the plugs and sensors check out, replace the coil pack.

Conversation Summary

A 1996 Pontiac Grand Am with a Quad 4 set codes P0420 and P0304, and the light returned in wet weather. The owner asked whether the coil housing could cause it. The expert advised checking plug 4 first for gap, fouling, and looseness, noting that retarded spark can also trip P0420. Next, test the rear Bank 1 O2 sensor and the other sensors; varying voltage indicates a bad catalytic converter. Consider a coil pack only if the plugs and sensors are good.

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