It seems as if all of these complaints are quite common with the Altima 2.5L (QR25DE) engine: code PO420, exhaust rattling, and seeing bluish smoke coming from your exhaust. All these symptoms can point to the same cause: a bad exhaust manifold/catalytic converter.

The factory catalytic converter is designed to last the lifetime of the vehicle. With no moving parts it’s only function is to convert toxic chemicals in the exhaust into less toxic substances. The automotive catalytic converter core is usually a ceramic monolith with a honeycomb structure. The Catalyst is most often platinum, this precious metal allows a reaction that converts carbon monoxide (CO) and un-burned hydrocarbons (HC), and a reduction reaction converts oxides of nitrogen (NOx) to produce carbon dioxide (CO2), nitrogen (N2), and water (H2O). Failure of the catalytic converter is most-likely due to a fuel management problem, faulty ignition system or from chemical contamination like burning oil (the bluish smoke from the exhaust).

*Tech Note- Bluish smoke coming from your exhaust when you start the car or under acceleration is from burning oil. Typical oil consumption conditions are caused by worn engine parts like piston rings and valve guides/stem seals or a malfunctioning PCV system.

Why is this condition so common???? Nissan has done several Service Campaign Bulletins on this problem. If You’re experiencing a check engine light with code PO420 (3-way catalyst function) Nissan recommends contacting your local dealer to see if your ECM has the most current reprogramming. The code may be false, and the converter fine, the problem may actually just be as simple as a software update.

This Nissan 2.5L (QR25DE) engine is most-likely running at the borderline of its efficiency threshold. The exhaust gases are VERY hot right out of the engine, making the possibility of damaging the pre-catalyst. When these extreme conditions are present the pre-cat gets burnt, but why it’s getting burnt is debatable. Were these conditions caused by unburnt fuel from bad combustion due to fuel management or a fault ignition system. Or from excessive oil consumption, that bluish smoke from the exhaust system caused from worn engine parts. As the catalyst media break apart, the engine back-pressure under acceleration pulls these pieces into the combustion chamber. This catalyst material damages the piston rings resulting in increased oil consumption causing the problems to magnify.

Replacing the exhaust manifold/catalytic converter has a estimated labor time of 2.0 hours and is a very straight forward repair. However there are a few points of interest here. First, there is minimal room to get a wrench on the serpentine belt tensioner making a simple belt removal frustrating. Second, be careful with the manifold nuts/studs as they are know to break (weakened from excessive heat).  Lastly, the O2 sensors can be extremely difficult to remove. Think leverage and use a substantial breaker bar or extension to gain enough mechanical advantage.



***First Check Vehicle for California Emissions, the label is located under the hood.


Removal and Installation REMOVAL 1) Disconnect battery. 2) Remove engine cover.

3) Remove the serpentine belt.


4) Disconnect alternator wiring and remove the alternator.

5) Disconnect the electrical connector of each heated oxygen sensor, and unhook the harness from the bracket and middle clamp on the cover. Remove the heated oxygen sensors.


6) Remove the lower exhaust manifold cover. 7) Remove the exhaust front tube, connected to the lower side of the manifold/cat. *Spraying rust penetrant on exhaust bolts will help with disassembling these parts. 8 ) Remove the upper exhaust manifold cover.

9) Loosen the nuts in the sequence shown, on the exhaust manifold and three way catalyst.


10) Remove the exhaust manifold and three way catalyst assembly and gasket.

Installation is in the reverse order of removal. Pay attention to the following.

Tightening Exhaust Manifold Nuts -Tighten the nuts in the numerical order shown, Torque manifold to 29-32 Ft. Lbs. After tightening No. 5 retighten No. 1 and then No. 3 to specification.


*Installation of Air Fuel A/F Ratio Sensors and Heated Oxygen Sensors
Clean the A/F sensors and heated oxygen sensor threads, then apply the anti-seize lubricant to the threads before installing the A/F sensors or heated oxygen sensors.

CAUTION: Do not over-tighten the A/F sensors or heated oxygen sensors. Doing so may cause damage to the A/F sensors or heated oxygen sensors, resulting in a malfunction and the MIL coming on.

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