02 Sensor CEL Code 42

[NSX-Files]

I just installed new Comptech headers in my 1991 along with brand new NTK O2 sensors from SOS (the original ones in the manifolds were really tight - broke an O2 sensor tool trying to remove it - so I took the lazy way and bought new sensors). I also installed a Dali Fresh Air Intake Scoop at the same time. I already had a Comptech muffler and UNI filter.

Before running for the first time, I reset the ECU according to the manual to let it "learn" the new components. A few seconds after every start up, I get a CEL. After pulling the OBD1 code, it's "42" which is the rear O2 sensor heater.

The car seems to run fine although it smells a little rich and is lumpy at idle. Would a 42 code cause it to run in open loop mode?

Could I have gotten a faulty new O2 sensor? I'm using the O2 sensor extension wires that came with the Comptech headers. I did a search and noted a post from Chris at SOS regarding crossed wires in DC header extension leads. Could that be the problem? I haven't yet tested the resistance of the terminals according to the manual, but am looking for leads to diagnose the problem.

 

[Daedalus]

It's not crossed wires. The pre-cat O2 sensors have 2 internal circuits. 1 circuit is for feedback on O2 levels. The other circuit is just a heater to heat up the sensor faster. Without the heater the car will run rich longer until the exhaust heats it up enough. Check the connectors for bent pins.

 

[NSX-Files]

It's not crossed wires. The pre-cat O2 sensors have 2 internal circuits. 1 circuit is for feedback on O2 levels. The other circuit is just a heater to heat up the sensor faster. Without the heater the car will run rich longer until the exhaust heats it up enough. Check the connectors for bent pins.

Thanks for the response. Actually, it was crossed wires in the Comptech extension harness. After testing the resistance of the sensor itself according to the service manual, it had the proper 10-20 ohms. However, when I tested the leads at the other end of the O2 extension wires, I found the pins were in the wrong place. The heater circuit wires were where the O2 sensor wires should be. Basically, the group of 4 wires were rotated 180 degrees.

I took the extension harness connector apart and switched the wires to where they should be. Presto! That cured the problem. No more CEL and it does not smell rich anymore. The idle is totally smooth at 850 rpm and overall it's running perfect.

Just seat-of-the-pants, it's obvious that headers are by far the biggest performance improvement to be had for a 3.0 NA. It revs so much quicker. I really need to dyno this thing to see what it's got.

 

[Daedalus]

Sorry, when you said crossed wires I thought you meant the extension was plugged in to the wrong harness. That has happened before as well. Glad you got it figured out fast.

 

[Larry Bastanza]

If you purchased from them new, please make sure you let Comptech know that this is an issue, so they check them all.

Regards,
LarryB