Question: I have a 1996 Chevy C1500 pickup with a 5.7-liter. I recently had to replace the old CSFI unit with the upgraded MFI due to a misfiring problem that wouldn't go away. After the upgrade, it ran great for about a week, and then I got another SES light. AutoZone said it was a Cat Converter failure in Bank 1, possibly caused by a large vacuum leak or bad O2 sensor. I noticed a strong gas odor coming from the engine and that the EGR valve had a line that looked like it had been sheared at the elbow and now goes to nowhere. It doesn't seem to be pulling any vacuum, and I can't find where the line is supposed to go. Are there any diagrams available? Could this be the problem, or should I look elsewhere?
Answer: It sounds like Trouble Code P0420: TWC (three-way cat) System--Low Efficiency Bank 1. Regardless of the MFI conversion, you still want to cover the basics. There are faults other than the catalytic converter itself that can cause P0420, and typically there will be additional trouble codes present (O2 sensor or misfire related), which should be diagnosed first, before condemning the cat. There's one O2 sensor before the catalytic converter and one after. The two sensors determine the catalytic converter's efficiency by measuring oxygen content before and after the exhaust gases pass through the converter. The smell of fuel and a vacuum fitting disconnected (EGR) are red flags and should be dealt with before anything else. If you smell raw gas under the hood, there's a fuel leak that must be fixed first. A service manual will provide a vacuum diagram to help determine if that open port is actually an issue. You might want to check with ALLDATA, www.alldatadiy.com, which offers vehicle-specific online service information at monthly and yearly rates. After checking the basics, bang on the catalytic converter and listen for anything rattling around--a sign the converter is coming apart inside and can no longer do its job. Also look for any physical damage or exhaust leaks anywhere in the exhaust system. If you run out of options, get it to a dealership or a good independent shop for an accurate diagnosis, if nothing else.
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If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 6420 Wilshire Blvd., Los Angeles, CA 90048, or e-mail us at trucktrend@sourceinterlink.com. Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.