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Q: cat/O2 sensor/MAF sensor/compression check on 2001 Nissan Sentra

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Check Engine Light came on accompanied by rough idle and 3 shops told me Cat replacement was needed. Estimates all close if not exact. Went to the one closest to home and their analysis concluded that cat failed due to bad O2 sensors. Replaced Cat and O2 sensors. Light came back on within a few days. Returned and they reset the light and said to watch it for a while, they would replace Cat if light came on again. No further light, but rough idle getting worse. Returned to have them check it again, and they said problem was 2 cylinders with very low compression, possible valve job required. Took it back to one of the original 3 shops and they saw no indication of compression problem (didn't do a compression check, just saw no signs). They could see new Cat/O2 sensors and assumed those were OK. MAF readings extremely low, so replaced. Rough idle continued and further check by them indicated "FUEL TRIM READINGS ARE VERY LEAN. THIS IS FROM THE O2 SENSORS AFTER THE CAT GIVING FALSE READINGS DUE TO THE CAT". They suggested cat replacement may have been "less than OEM quality" and that I should return to the shop that installed it and ask for a replacement. $1563.69 so far, and other than no current check engine light, no improvement. What am I missing and what should I do next?
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I want to know what the compression is. Did they actually take a compression test and give you the results? it'd be good to have more information. If the car runs rough, maybe it's misfiring and there should be a trouble code stored in the computer if so. Also, what codes were present when the shop diagnosed the car. It's possible to get a less-than-adequate replacement catalytic converter. I'd also look for an air leak into the air intake system. If the compression is in fact low, you may get by with a valve adjustment rather than a full valve job. A 'leak-down' test is advisable to make sure you know where the lost compression is. Valve clearance must be checked periodically and adjusted if necessary on your car.
Here's a direcrtory link for you.
The shop that replaced the catalytic converter and O2 sensors did a compression check with these results: #1 cyl 120 psi, #2 cyl 95 psi/, #3 cyl 85 psi, and #4 cyl 135 psi. The shop that replaced the MAF sensor "erased pending code PO430, and code came back after test drive." I don't know what the numbers were for the codes that were present when the "check engine" light was on, but 3 different shops told me they indicated a failed catalytic converter. The catalytic converter shop assured me their part was "factory quality." No one has said anything about an air leak in the air intake system. Catalytic converter shop said "leak down test done will need head work." Does this information clarify anything?
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