I went to buy an 07 Nissan Xterra SE and it had a Check engine light for a cylinder 1 misfire. So the guy took it to his buddies mechanic shop, and they replaced the one spark plug and coil. Retest drove it and seemed fine, so I bought the car and started driving home.

Within a 30 minute drive the car was driving rough. The engine felt as if it might have been rumbling or shaking a bit so I pulled over. I had a Service engine soon light on and had it coded, the code I got was p0420 catalyst efficiency below threshold. After looking things up I do believe this happens a lot for this car and year - just the Catalytic converter going out.

So the guy took the car back to the mechanic and the first go round when they took the intake manifold off to get to cylinder 1, they might not have put it back on correctly and I feel cold air coming out. So I believe there is a vacuum leak...

It was at the shop and the guy told me that he bought a new Intake manifold for them to put on but I found out he lied and when I spoke to the mechanic he said he never received it so all they did was replace the brand new coil pack on the cylinder 1 spark plug and it supposedly fixed it and made the light go off - until I drove it home and the light came back on.

It doesn't seem to be running rough but I'm not sure if the engine only have 1 new plug and coil is throwing off the balance and making the code for the catalytic converter show up or if its due to the catalytic converter going bad or if its the intake manifold having a vacuum leak. Any thoughts?

  • My thoughts are that I wouldn't trust the person you're buying the car from (or his 'mechanic' friend), so I'd move on to another. Don't fight the issues.
    – PeteCon
    Apr 21, 2017 at 5:12

2 Answers 2


p0420 is caused by a damaged catalytic converter or O2 sensor. Since you said that the vehicle had a misfire code, they most likely drove with the light on because they figured they would sell it. The Catalytic Converter is a giant oven that stores and releases oxygen. Dumping fuel, from the misfire, destroys the honeycomb inside the cat which contains precious metals which is why its expensive to replace.

Find another vehicle. Its going to cost alot to replace the catalytic converter and mostlikely has other issues.

If you do replace it, only get a dealer converter. After market cats dont use the same metals as the orginal manufacture. Could be also an O2 but would lean more towards cat because of misfire.


The catalytic converter is build to last the life time of the car. The P0420 is shown when the cat efficiency goes under 95% so I wouldn't replace it directly (costs between 300-1000$).

Also the catalytic converter is determined deficient if the O2 sensors post and pre-cat are not working right or giving similar readings which means the cat is not making a difference.

On the other hand, missfiring can be cased by a clogged catalyst or malfunctioning O2 sensor.

Malfuncitoning O2 sensor can tell the ECU that the Oxygen is too high, making the ECU pumps more fuel and vice versa, that may directly causes missfire.

You can confirm it is not a coil or cable that causes the missfire if you reset the error code and the engine runs smoothly after.

This car is older than 10 years, it might be time to replace the O2 sensors, they should be changed every 150,000km depends on the car.

This is just one possible scenario and it could only get worse, If you can, return the car.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .