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The P0113 code is for a IAC indicates the temperature below -40 degrees. Possible causes for this code are a faulty sensor, a faulty harness connector, or a faulty PCM. The P1661 code is indicating that the output circuit check for the PCM is low. The causes of this code are low battery voltage, VREF signal faulty, or a faulty PCM. The P0476 code is for the exhaust pressure control valve performance. This code indicates a malfunction with the Exhaust Back Pressure regulator. The causes for this are a stuck or damaged butterfly, misadjusted or damaged linkage, restricted exhaust, exhaust leaks, a faulty EPR, faulty back pressure sensor (EBF), or a faulty PCM. I would double check to make sure the P0476 is the only one coming up. I have a few tests that you can perform. This will require a digital voltmeter that is also able to measure resistance. Let me know which direction you want to go and I can assist you further. If you have any questions, feel free to ask. Thanks.
If your mechanic is equipped with the proper scan tool and software these test can be performed with the scanner. The first tests are for the P0476. The second tests are for the P1661. Sorry I was unable to post the tests as a whole so I had to post them in sections. They are numbered in order. Let me know how you make out. Thanks.