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I have a Honda Civic 1.4 on a 'W' plate (2000)
A month ago my engine management light came on so I took it to a garage for a diagnostic check. They told me that the error code did not refer to any particular problem so they reset the circuit and every thing seemed okay until last week when the light came on again after I had been traveling on a longer distant (50 miles). I booked the car in to Honda and they told me the code was PO172 which they said indicated the number 1 O2 sensor and/or the 'Cat' was the problem. I bought a new sensor and got the local garage to fit it and perform a 'Cat' test at the same time and it passed the emissions test, however on leaving the garage the light came back on after about a mile or so. I have been told by Honda that there is a secondary sensor after the 'Cat' but I am unsure whether this is the problem as surely it wouldn't pass the emissions test if it was. Another thing which is unusual is the fact that my old sensor doesn't display any problem until I have traveled a longer distance i.e 50/60 miles but the new sensor shows a problem after a mile. The car in running seems okay as the only real symptoms are rougher idling, more accelerator pedal from standing and a slight increase in fuel consumption. If anyone has experienced the same issues or someone knows what else could be the matter I would be grateful to find out before spending a load more money on diagnostics and sensors.
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If you have two sensors...their role is to not only test emissions but to test if the CAT is blocked. If the CAT is not blocked....(have you lost power)....I would supect that air is getting in the system from somewhere other then th exhaust....ie you may have a leaking mainfold gasket or crack in the manifold before the CAT.

If you have a leak it will think the mixture is too lean and increase the fuelling to match the perfect air/fuel ratio. So your lambdas will say lean (due to extra oxygen) but your engine will be running rich and lose power. Pull the spark plugs and they will probably be black soot coated instead of brown.

I have a Honda Civic 1.4 on a 'W' plate (2000)
A month ago my engine management light came on so I took it to a garage for a diagnostic check. They told me that the error code did not refer to any particular problem so they reset the circuit and every thing seemed okay until last week when the light came on again after I had been traveling on a longer distant (50 miles). I booked the car in to Honda and they told me the code was PO172 which they said indicated the number 1 O2 sensor and/or the 'Cat' was the problem. I bought a new sensor and got the local garage to fit it and perform a 'Cat' test at the same time and it passed the emissions test, however on leaving the garage the light came back on after about a mile or so. I have been told by Honda that there is a secondary sensor after the 'Cat' but I am unsure whether this is the problem as surely it wouldn't pass the emissions test if it was. Another thing which is unusual is the fact that my old sensor doesn't display any problem until I have traveled a longer distance i.e 50/60 miles but the new sensor shows a problem after a mile. The car in running seems okay as the only real symptoms are rougher idling, more accelerator pedal from standing and a slight increase in fuel consumption. If anyone has experienced the same issues or someone knows what else could be the matter I would be grateful to find out before spending a load more money on diagnostics and sensors.
Thanks
 

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if there has been an air leak for a while and the car has compensated by running ritch it is possible that the cat is not working as effeciantly as it should. but passing the emmisions test would seam to rule this out. the new sensor will be more sensitive than the old due to it having run less miles than the old one so may explain why it is sensing a problem sooner. too much or to little oxygen is exiting the engine and as menchoned previously check the spark plugs condition, air filter, any vacuum leaks or leaks from either manifold. hope this helps
 
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