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Without the O2 sensor, the ECU goes into open loop mode.Did you separate the heater ground from the sensor ground? Having them together can affect the O2 signal (especially if your power source for the heater is on the coil, since the voltage will vary with the ignition) This variance will make it to the heater ground wire, and will overwhelm the O2 sensor signal output if it's grounded with the heater ground. (the sensor ground will vary up and down by as much as 100 milliamps, which will cause the O2 sensor signal to vary that much; which will confuse the ECU if the output voltage goes up or down when it's expecting down and up)
I digital voltmeter will work at lower speeds. It flip-flops to fast for the meter when you're going faster.Did you pull the fuse to the ECU so it can start over with the relearning?
I'd guess that when the CEL goes on, it's running in open loop mode because it thinks the O2 signal is invalid. Open loop is usually richer.When the CEL turns off, it thinks the O2 sensor signal is once again valid, and runs in closed loop. (leaner).
Is the heater in the O2 sensor good? What's the resistance across the heater wires?What about an exhaust leak at the head/header flange? Bad weld in header? EGR holes in head/header flange?
About the EGR. Wouldn't I get a different code other than Code 5 to tell me there is a problem with the EGR?
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