1997 Saturn SC2 trouble codes
 

I'm getting the following codes and poor gas mileage. Any ideas on how to fix them? I have inspection coming up and will not pass w/ check engine light on.

P0141
P0340
p0300

P0300 -- random/multiple misfire detected
P0340---camshaft position sensor circuit
P0141--O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)
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P0141 -- Rear (post Cat) 02 sensor likely defective -- replace -- this one WILL cause bad fuel economy

P0300/P0340 -- there is no camshaft sensor on the s cars; 0340 on an s car usually indicates an ignition related issue...set with 0300 (random/multiple misfire) kinda confirms this....

for misfire codes that point to a particular cyl (P0301-P0304), it's often just a single bad spark plug wire; but a 0300 code with no addl specific cyl code leads me to believe it is either that the car is in need of a tune up (new plugs and wires all around), low compression across the cylinders (worn rings. head gasket), an issue with one or both of the coil packs malfunctioning, or a DIS (ignition module) issue.
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I'd start with replacing the wire set and plugs with stock Saturn NGK plugs. If that doesn't solve your ignition codes, I'd do the compression check first...if it is low compression, you've got to address the compression issue to address these codes.

Be sure to do with a warmed up engine, PULL the fuel injector fuse (to avoid spraying fuel into the cyls while doing the compression check, and have all plugs removed.

Please post results by cylinder and we'll go from there

Actually you can check out the coils now too.......DIS is a bit more of a pain