Thanks for everyones input. I guess its safe to assume the mechanics at the dealer should know which cat to change if they read the code saying basically bad cat bank 1 (p0420 catalyst below threshhold bank one). I just remember going through this with the 02 sensors and the first mechanic I talked to telling me that the sensor in the front was bank one.
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Yeah right :rolleyes2:
Bank 1 and 2 are also reffered to as "right hand and left hand". If the engine were inline with the vehicle, meaning the accessory belt / crankshaft pulley side faced the front of the vehicle, and the output / flywheel side faced the rear, bank 1 would be on the right, bank 2 on the left, from the driver's seat point of view. At least with Ford, when the engine is horizontally opposed for FWD applications, the bank reference does not change. Bank 1 = Right Hand, Bank 2 = Left Hand. The right hand bank is against the firewall, left hand behind the radiator.
So when the mechanic told you P0420 referred to the front bank, he was wrong.... However, he may have meant the upstream O2 sensor.... which again, he would be wrong. There are 3 cats on exhaust and 4 O2 sensors. There is one cat for each bank, and a 3rd cat located after Bank 1 and 2 exhaust merge. The 3rd cat does not have a sensor. Upstream sensors are sometimes called the "front", while downstream sensors the "rear". The correct way to identify the sensors is to label it Bank 1 Sensor 1 (right hand upstream) or Bank 2 Sensor 2 (left hand downstream), etc.
P0420 refers to Bank 1 Sensor 2 (right hand downstream). The downstream (aka "rear") sensor's purpose is to monitor the efficiency of the cat. If a cat goes bad, you'll get P0420 for Bank 1, and P0430 for Bank 2. Simetimes it is the sensor that has gone bad. The sensor gets "lazy" with age. The PCM references the sensor switching frequency. If the sensor switches slow, it sets a code. Usually this means the cat is bad, but if the sensor is old, it may be the sensor at fault. A diagnostic scanner will report the sensor switching speed. Compare that data with a tail pipe sniff test (emission tester). If the emissions are withing spec, the cat is working, if the cat has failed, emissions will be high and out of spec. It is much less expensive to replace an O2 sensor than a cat, so start there.