Strange Driveability Problems on a 2002 Nissan Sentra

I recently had the opportunity to work on a 2002 Nissan Sentra that had some strange symptoms. For just the solution, you can skip to the last two paragraphs. 

Here is a list of the problems:

• Power loss at part to mid throttle
• Hesitation
• Rough idle
• Random misfire on multiple cylinders
• Hard shifts at part throttle
• But it still had full power at full throttle

I got involved last spring when a neighbor needed help with a misfire. My OBD-II scanner showed it had a crankshaft position sensor code. I figured this was going to be an easy fix. I put a new crank sensor in and thought that was it. Nope. The check engine light came on soon after. This time it was for catalyst efficiency below threshold. 

The misfire situation melted the catalytic converter (as I discovered later). Just replacing the crankshaft position sensor didn't solve the strange driveablility problems listed above, so I cleaned the mass airflow sensor and checked the engine for signs of any vacuum leaks. I think I remember changing the spark plugs, but my neighbor thinks we didn't. 

I figured that since the car had 200k miles on it, it was time to replace the upstream oxygen sensors. That didn't help. I finally ran an exhaust back-pressure test and saw that the pressure was high. That in combination with the catalyst efficiency codes led us to replace the converter. The borescope images weren't really good, but showed what appeared to be melted honeycomb. 

Here's what it looked like after I got it off the car. 

Melted catalytic converter

After replacing the catalytic converter, I was careful to put the oxygen sensors back in the same places that they were in before. Even though the plugs were color coded and wouldn't fit in the wrong plug, I had a feeling in the back of my mind that it would still be possible to swap the oxygen sensors on the exhaust by installing them in the wrong hole. But I figured that if I was just careful not to swap them, we'd be ok. 

With the new converter, the car still had the same driveability problems as before. I was beginning to worry at this point. 

By the way, the new converter didn't match-up exactly on the lower mounting bolts. One hole was over a centimeter off, so I couldn't get the bolt in, but it'll work and pass inspection. 

If the engine was worn out, nothing else would matter much, so for a sanity check, I ran a compression test. Here are the results:

Cylinder 1: 122 PSI

Cylinder 2: 117 PSI

Cylinder 3: 120 PSI

Cylinder 4: 121 PSI

I cranked it with all the plugs in, but all the ignition coils removed. I cranked it on each cylinder for about eight compression strokes (until there was no more increase on the gauge). When re-installing the plugs, I used anti-seize compound on the spark plug threads, and dielectric tune-up grease on the inside of the plug boots. I Cleaned the MAF again, looked for vacuum leaks, and took it for a test drive. 

It should have run well, but there was no improvement. 

I connected my OBD-II scanner to it and saw that one bank's oxygen sensor was lean, and the other was rich. As the computer further leaned out one bank, and further enriched the other, I realized what the problem was. The upstream oxygen sensors were swapped. The blue connectors are for bank 1, and the green plugs are for bank 2. The outside two connectors are for the upstream sensors, while the inside two are for the downstream sensors. 

A quick google search got me the verification I wanted. Indeed, the upstream oxygen sensors were swapped. I put them in their correct places and took it for a test drive. Although it drove like a 14 year old car that needed shocks and struts, the engine was back to normal. 

I have no idea when the sensors got swapped, or if it was even my fault. I didn't verify that the sensors were correct, so that's on me. I just went off of where they were before. But at least it's fixed now.