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Old 11-29-2010, 11:00 AM   #1 (permalink)
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Default Poor MPG help??? P0420 Catalyst Efficiency Below Threshold

Hi I'm new to the forum so hello to everyone I've recently purchased a 52 plate Suzuki Jimny Special hard top version. The malfunction check light has come on which reads error code P0420 Catalyst Efficiency Below Threshold. My journey to work is 75 miles from Wakefield - Whitby at 50-60mph. Fuel consumption is an alarming 17mpg. My question is which part do I need to replace if anyone can help as I'm struggling to pay for the fuel. My investigations lead me to believe that it is one of the Lambda Sensors or the Cat itself that needs replacing. Any help would be greatly appreciated. Chris
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Old 11-29-2010, 11:15 AM   #2 (permalink)
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hopefully it's just a sensor, those are easy and cheap to replace. If it's the catalytic converter then you got an expensive repair. Whether it's the sensor or the CAT failing, you need to replace it ASAP. Excessive Backpressure from a failing cat will cause the combustion chamber temperatures to rise to the the point where it will warp the head and blow the head gasket. You'll notice a significant power loss from a blocked CAT though, not just poor fuel efficiency. If it's the O2 sensor, and you are running too rich or too lean, the poor fuel mixture will overload the cat and cause it to fail prematurely. Easiest thing to do is replace the O2 sensor and clear the code. See if it throws the code again. If it does, then you know the CAT is going.
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Old 11-29-2010, 11:51 AM   #3 (permalink)
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Hi Baratacus, many thanks for your quick reply. I agree and understand with what you're saying. Better to replace the O2 sensor first and then the CAT if necessary. Which sensor would need replacing as there are two I believe. One pre and one post CAT?
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Old 11-29-2010, 05:58 PM   #4 (permalink)
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the front one is what the vehicle uses to control the fuel mixture. The downstream one (after the cat) is the one it uses to determine if the CAT is functioning correctly. If you have a trouble code telling you the cat is bad, then it's being sent by the downstream o2 sensor. If you replace the downstream sensor and you still get the indicator that the CAT is bad, you should replace the CAT AND the upstream sensor. A lot of times a failing catalytic converter is a result of poor fuel mixture causing it to be overtasked and over heated, breaking down it's catalytic components. You could put in a new cat, and the upstream sensor could still be causing a fuel mixture imballance that will be overworking the new cat and cause it's premature failure. That would also be a good time to do a tuneup replacing the plugs, wires and coil packs and servicing the throttle body.

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Old 12-06-2010, 03:43 PM   #5 (permalink)
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With that fuel economy, I'd expect that the mixture is pretty rich, which can kill a cat in short order. A bad cat code will not cause a fuel economy hit, though a clogged cat certainly can. I'd replace the front sensor as a maintenance item.

I had the cats on my '96 Impala SS go out after about 170,000 miles. Same code, one going bad and then the other about 3,000 miles later. I've never replaced the rear sensors, because the connectors are in such a bad place, and they have no effect on the mixture. They also sit in a pretty clean stream, so are not subject to the contamination issues of the front sensors. I put two low mileage cats in and all is well. Still using the OEM rear sensors at 180,000 miles. I'm now on my 3rd set of front sensors.
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Old 12-07-2010, 08:54 AM   #6 (permalink)
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good to know. With my samurai a sensor cost me about 15 bucks. I'd just replace all of them at that cost. With the newer jimny the dang sensors are around $200 for both. ($75 rear and $125 front) I'd replace as few sensors that I can get away with at that price. CATs are rediculously priced as well for later models.
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Old 12-07-2010, 11:09 AM   #7 (permalink)
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It's even worse in California. That is why I put low mileage used cats in my Impala, rather than new ones.

A pair of 50,000 mile cats set me back $150 shipped, vs $600 plus shipping for a set of new ones. I figure the used ones are good for at least another 100,000 miles. If there was a performance benefit, I'd of probably went with new, but the Impala came with high-flow cats already.

The Samurai's California cat was only $130, but a 49 state cat would have been half that.
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