Sounds like your heaters are good. Here's the text from my Bentley:
Oxygen Sensor O2S Heater, Checking
Observe all safety precautions: Safety Precautions
View clean working conditions: Clean Working Conditions
Use only gold-plated terminals when servicing any component with gold-plated electrical harness connector terminals.
For wiring diagrams, component locations, and connector views, Refer to the applicable wiring diagram.
Oxygen Sensor (O2S) Heater Z19 is part of Heated Oxygen Sensor (HO2S) G39 and cannot be replaced separately.
Special tools, testers and auxiliary items required
multimeter.
Wiring diagram.
Test requirements
Fuse SB15 OK.
Motronic Engine Control Module (ECM) Power Supply Relay J271 OK.
Battery voltage at least 12.5 volts.
All electrical consumers such as, lights and rear window defroster, switched off.
Vehicles with automatic transmission, shift selector lever into position "P" or "N" .
A/C switched off.
Ground (GND) connections between engine/transmission/chassis OK.
Ignition switched off.
Test procedure
Perform a preliminary check to verify the customers complaint. Refer to Preliminary Check .
Start diagnosis
Remove the engine cover with air filter. Refer to the appropriate service manual.
Checking internal resistance
Disconnect the Heated Oxygen Sensor (HO2S) G39 electrical harness connector - 1 - .
Using a multimeter, check the Heated Oxygen Sensor (HO2S) G39 terminals 3 to 4 for resistance.
Specified value: 2.5 to 10.0 Ω (at approx. 20 degree C)
If the specification was not obtained:
Replace the Heated Oxygen Sensor (HO2S) G39 . Refer to the appropriate service manual.
If the specification was obtained:
Checking voltage supply
Crank the engine (engine may run).
Using a multimeter, check the Heated Oxygen Sensor (HO2S) G39 electrical harness connector terminal 4 to Ground (GND) for voltage.
Heated Oxygen Sensor (HO2S) G39 electrical harness connector terminal Engine Control Module (ECM) J623 electrical harness connector T94 terminal or test box socket
4 Ground (GND)
Switch the ignition off.
Specified value: Battery voltage.
If the specification was not obtained:
Check the Heated Oxygen Sensor (HO2S) G39 electrical harness connector terminal 4 to the Motronic Engine Control Module (ECM) Power Supply Relay J271 socket 2/87 an open circuit, a short circuit to each other, Battery (+), or Ground (GND).
Check the electrical harness connector for damage, corrosion, loose or broken terminals.
If necessary, repair the faulty wiring connection.
If the specification was obtained:
Checking Ground (GND) activation
If the manufacturers test box is being used, perform the following step.
Install the test box.
If the manufacturers test box is not being used, perform the following step.
Remove the Engine Control Module (ECM) J623 . Refer to the appropriate service manual.
Using a multimeter, check the Heated Oxygen Sensor (HO2S) G39 electrical harness connector terminal 3 to the Engine Control Module (ECM) J623 electrical harness connector T94 terminal 73 for resistance.
Heated Oxygen Sensor (HO2S) G39 electrical harness connector terminal Engine Control Module (ECM) J623 electrical harness connector T94 terminal or test box socket
3 73
Specified value: 1.5 Ω max.
If the specification was not obtained:
Check the wiring for a short circuit to each other, Battery (+), and Ground (GND).
Check the electrical harness connector for damage, corrosion, loose or broken terminals.
If necessary, repair the faulty wiring connection.
If no malfunction is detected in the wiring and if the voltage supply was OK:
Erase the DTC memory. Refer to Diagnostic Mode 04 - Erase DTC Memory .
Perform a road test to verify repair.
If the DTC does not return:
Generate readiness code. Refer to Readiness Code .
If the DTC does return and no malfunction is detected in the wiring and the voltage supply was OK:
Replace the Engine Control Module (ECM) J623 . Refer to the appropriate service manual.
Final procedures
After the repair work, the following work steps must be performed in the following sequence:
Check the DTC memory. Refer to .
If necessary, erase the DTC memory. Refer to .
If the DTC memory was erased, generate readiness code. Refer to
