Chevy & GMC Vans

I can't solve this problem... The computer keeps bring up code 13. Engine light comes on then goes out, goes on, then goes out but it's not timed or predictable. It comes on when it wants to. It's saying that the Oxygen sensor is at fault but in fact it's fine.
- We checked the TPS with a volt meter, and it was running normal (all though it wasn't through a code at the time).
- We tested the wires going to the Oxygen sensor and it was showing a reading as if the engine was in open loop.
- We also tested the injectors and there fine.
My engine is running very very rich (so bad even I can smell it). I've got 157, 000k on my 93 Z engine. I don't think it's the ECM because this happened with a different ECM.

Steph

The coolant temp switch if its reading cold will make it run rich , try that steph .

check the coolant temp sensor first, but my thought is if the injectors are original my bet is that one or both of them are beginning to stick full open. pull the air cleaner housing and the upper part of the throttle body and turn the key. watch the spray pattern on the injectors, if it doesn't mist in a cone and drips or pees into the intake you have a bad injector.
my other recommendation is if you have to replace an injector is to buy a gm oem injector, new. I did the online rebuilt thing and regretted it big time. they are expensive as hell but they'll last another 100K at least.

The lean (44) & rich (45) codes set when the voltage on the circuit is stuck low/lean or high/rich too long.
Code 13 is typically set when the O2 sensor circuit voltage does not move when it should. This can be either a open or short to the circuit. Check the harness to the sensor carefully for any chaffing to ground or poor connection to the O2 sensor. This code will also set if the sensor becomes inactive because of an internal fault. If you were to monitor the voltage on the O2 sensor signal wire, you would typically have 450 millivolts on the wire. As you drive/run the engine it should vary in the range of 100 - 900 millivolts. The computer will set the code 13 if it reverts to 450 millivolts.
Hope this helps, James.

Thanks every one for the help! When Sanja took the main wiring harness appart we discovered that someone was in there and had cut/ stripped wires and purely put them back with duck tape... Since I know this van from day one, I know that the van has been to some shabby shops and this is their "finished work"...
What you said James sounds exactly that. So I'm going to go wire hunting, I wouldn't be surprised if I find more wires in bad shape.
Thanks again!!