94 Audi 100 S "Check ECS" light

I'd look for a dodgy earth or perhaps a damaged wiring loom to the ECU.

I had O2 sensop replaced twice in error (by different garages) on my son's BMW 325. Finally it turned out to be poor crimping of the harness connector at the sensor end. Before you proceed with replaceing the sensor a word of explanation: The sensor could be tested on the spot or after removal. The best approach is to use a break out box. I made one and presented it to the garage that wrongfully replaced the sensor the second time )after being told it was already replaced). Unplug the harness from the sensor. Find in the electronic shop nearby a set of matching male and female pins and solder them in pairs to 4 shor wires. Now restore the connection through this grude "break out box". You just gained access to the pins live during operation. After warming the engine for a while, the engine controller should apply 12V to the heater (its other end to GND). You can check this by digital multimeter or small bulb to chassis. Now the voltage across the other two terminals (one grounded again) should fluctuate around 0.45V. The sensor heater consumes over 1A initially and then drops to maybe half its original value, when it reaches operational temperature. To measure it you have to break that circuit (unplug one wire end) and restore it through an ampermeter. Here is how to test a sensor. Unplug the harness connector and measure the resistance between the wires. Identify the pair with less than 10 Ohm resistance (the heater). Check the harness connector mating connector pins for GND to one of them and 12 V coming after warming the engine. You have verified so far the heater operation. You can apply 12 V and GND to the identified heater wires and check the current without the engine controller (or engine running). Once it is stabilized you can measure the voltage across the other two pins. If it is removed and in your hand you can even exhale against it and see the change in voltage. Electrically the sensor comprises two parts - heater and actual sensor. They may have two independent wire pairs (one end grounded in each) or share a grounding wire. Remenber one heater end is normally grounded. One actual sensor end is grounded too. If you end up replacing the sensor - get a generic Bosch sensor say from German and Swedish and splice it to the connector wires yourself (saving in the process half the cost). Normally the package comes with lieflet of instructions and even maybe some crimping ferrules. Good and luck. Plamen, Toronto P.S please let me know of your progress.

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