I'm still investigating the potential problem with the o2 sensor on a 1.4 polo. See here.

Tonights first question is "what resistance should the o2 sensors be?"

I did a log on a 30 minute journey, (see attachment), and while the fault didn't occur during the trip, the pre-cat o2 sensor seems erratic (jumps around at 65-85, peaking at over 300 ohms). Should it be like this?

The post cat one is rock solid at 15-17 ohms.

The other question is "should the o2 sensor heater circuit turn off?"

Reading around the subject I was expecting the heating elements to switch off after the car was up to temperature, but according to the log they both came on when the engine was started, and stayed on :confused: Is this usual?

I'd appreciate some thoughts.....

Thanks

30 min journey.zip

Oxygen sensor - Wikipedia, the free encyclopedia

not sure wether this is relevant to your fault as its based on what i've learned by working on Ford vehicles

The upstream O2 sensor will switch , what you're probably reading is it switching from weak to rich to weak etc , which is normal operation , as the sensor reads the exhaust gases it adjusts the fuel mixture , i.e. if the sensor reads lean it adds a bit of fuel etc

the downstream O2 sensor will just read a steady figure building up slightly then dropping down suddenly , it does this because the catalyst is "dumping" oxygen .

the heaters are there to get the sensors to operating temperature, from memory the heater element on the upstream O2 sensor comes on when the engine is started , then after the catalyst is warmed up enough and the emission system goes to closed loop , the downstream O2 sensor heater operates , i think the heaters stay on also .might be different on VAG cars though

as i said at the top of the post this is what the Ford petrol vehicles generally do

For a BBY engine -

Lambda value at 2850 - 2900 rpm 0,97 ... 1,03

Lambda probe version - Wide band probe (lambda)

technically speaking a lambda probe isn't a resistor, in fact it's a variable capacitor, try looking up 'capacitive reactance' in an encylocpedia

The lambda heater circuits usually have a positive temperature characteristic, so the resistance increases as the temperarure increases. Hence the heating current drops for a constant input voltage.

As you have a fault code for the heater circuit and have logged variable resistances then I would change the sensor given this information as long as you're sure you have check out the wiring for poor connections, shorts etc.