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What's up guys. I scanned my buddies CEL on his 03 Rav4 after it came on during his drive home from work. It was throwing a P0420 (catalyst system efficiency below threshold bank 1), and code P0430 (catalyst system efficiency below threshold bank 2). I cleared the codes, and immediately checked the continuity of all 4 sensors. They are all good. I understand the possibility of a lazy sensor, so I cleaned the connectors and started the car again. It immediately threw another CEL, and this time there were 4 codes. The original P0420 and P0430, and now also P1310 (coil pack #3), and P0300 (multiple misfire detected). The car is running fine, no noticeable loss of power or gas mileage, or a rough idle. All 4 coils were replaced within the last year or so. I decided to check all of the coil packs anyway and they are fine. Within the last 3 years he has also installed new catalytic converters, 02 sensors, and a new ECM. The only thing I could think to check is the MAF sensor, and its clean and seems to be operating fine. Any ideas on what could be tricking the ECM into thinking that it's having these problems, or anybody ever seen anything similar? Any help is much appreciated, I'm stumped. Thanks a lot guys.
 

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Hi, yes I have a pretty good idea, I am a retired mechanic / CA smog tech.

Check for AC voltage at the battery with the engine running at idle and slightly above idle. Use a good DVOM like a Fluke set on AC volts. If you measure more than .5 VAC then it is very possible that the alternator has one or more bad diodes allowing this to occur. I learned this from an instructor at one of my many smog license update classes years ago. He said if ever we ran into a situation where there were random codes that make no sense to check for this.
I have seen this happen, not very often but I have seen it.
Please let us know what you find.
 

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Discussion Starter #3
Thanks for the fast response, that was a good idea. It looks like the alternator is good. I tested coil pack #3 again, and still looked good, but the connector felt slightly loose. I replaced it and the vehicle still runs fine, but now it's no longer throwing the misfire and coil pack #3 codes which is a plus. But I'm still getting the two codes for bank 1 and bank 2. At less than 3 years old, I highly doubt that the catalytic converter is bad. So any thoughts on where to go from here? Like I said, all 4 sensors tested good for continuity. Possible a lazy sensor that is throwing off the other sensors? The air fuel sensors are well over $150 a piece, so I'd prefer to diagnose a problem before replacing. Thanks so much.
 
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