Hello,
I have a 1990 300te with 185k miles. It just barely failed emissions recently (on hydrocarbons). Here are the current values:
THC (g/mi): 1.87 (max 1.20)
NOx (g/mi): 1.34 (max 3.00)
CO (g/mi): 3.7 (max 30.0)
CO2 (g/mi): 739.1

Here are the values from 2 years ago, when it passed:
THC (g/mi): 0.97 (max 1.20)
NOx (g/mi): 0.75 (max 3.00)
CO (g/mi): 4.4 (max 30.0)
CO2 (g/mi): 798.1

Notice how THC and NoX went up roughly 100%. I don't know if that says anything.

It wasn't throwing any check engine codes, and I proceeded to check the EHA current. This is where it gets interesting. At key on, engine off, the current is about +22mA. As I understand, it should be 20. When running it oscillates around +3-4mA, and the duty cycle on pin 3 of the X11 connector oscillates too, which tells me that it's running in closed loop. When I disconnect the oxygen sensor under the passenger side carpet, the EHA current does NOT go to zero, but rather stays around 3-4mA, but does not oscillate. If I give it some throttle it changes a bit. Duty cycle on pin 3 is at a steady 49.9 with the oxygen sensor disconnected.

On top of this, the EHA is wet with gas underneath, so it probably needs to be replaced. Could this explain the current discrepancies? Could this also be why my car didn't pass the emissions test? Are there any other signals that could cause the EHA to have current without the O2 sensor connected, or is it definitely because the unit is leaking?

Thank you very much.

Michael