# code p0725 and p0420 continues



## fer.b (Mar 6, 2020)

I have a 2006 nissan altima that has continuously had the codes p0725 and p0420 on and off. We have taken it to the mechanic and have done some repairs on our own but these two codes seem to come back and go away every few weeks. We already replaced the O2 sensors and the Crankshaft Position (CKP) Sensor. Not sure what to try next and the mechanics are really not that helpful(one told us to replace the fuel injector just to later find out that was not necessary or the problem so we ended up getting a code reader) 

Any help is appreciated!Thank you


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## smj999smj (Jan 1, 2006)

I don't believe the two codes are related. For the P0420 3-way catalyst code, if there are no other codes besides the P0725 and if there are no intake system or exhaust leaks or obvious engine running issues, the most likely cause is an inefficient, upstream, catalytic converter. The P0420 is triggered based on monitoring of the upstream and downstream oxygen sensors, which are place before and after the upstream catalytic converter, respectively. The ECM is not seeing enough change between the two sensors, which usually means the catalyst substrate (a.k.a. "honeycomb") is not properly breaking down the exhaust emissions from the engine. The fix would be to replace the upstream catalytic converter, which is also the exhaust manifold on this engine. Rockauto usually has pretty good prices for them, but if you live in California or a state that follows California emissions standards, then you will need to use either a genuine Nissan part or an aftermarket converter that is C.A.R.B. certified.
P0725 is a transmission code for the engine speed signal and the most likely cause is an open or shorted circuit. Essentially, the TCM (transmission control module) is not receiving a correct signal from the ECM (engine control module). There would be a number of tests that would have to be performed to isolate the problem and the diagnostics begin on page AT-127, in the automatic transmission section of the 2006 Altima factory service manual, which you can find at NICO Club's site.


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