rayman_2

I am thinking of buying a air/fuel meter, would it be better to run an after market sensor or to use the original one ?

if aftermarket what place would be best to mount the sensor ?

nom

Erm...
unless you're going wide-band (ie expensive), you may as well use the standard one. It's handy to have an extra sensor I suppose to make sure that the one the ECU is using is doing its job, but you'd probably need to drill a hole in the exhaust somewhere. And then have something to power the heater in the sensor.
It'd probably all get a bit complicated
Standard one is good enough for most, & the others go wide-band, which only rally hlps if you can map your ECU, otherwise I think it's probably overkill!

rayman_2

The one I am considering is

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=33553&item=2423593 670

nom

I think - but may be wrong here - that this is a calibrated narrow band system. Which means that 99% of the complex (read 'expensive') stuff is already handled in the scoob by the car - it just doesn't give a read-out. It's possible to attach a read-out, most not being calibrated, but some can be (the LambdaLink) although normally isn't supplied as such. I think BR Developments ones are pre-calibrated, though.
I can't be entirely sure as they don't give information on what the lambda probe is, but I think the 4 wire ones are usually of the heated narrow-band variety.
So you wouldn't get much more accuracy, if any, over a normal AFR meter unit.
Others may know the HalTech unit better, though