2008 TD4 with 155k miles. Been putting off the job for a while, changed my O2 sensor with Bosch unit, and changed catalytic converter with BM unit while I was at it.

Backstory: was getting below 20 mpg (in town) and 22 mpg mixed driving for past 6 months. Brake discs, pads, pistons, hoses, all full overhaul. Gained 1/2 mpg (so there may have been some sticking caliper). Next in order, more ‘preventive maintenance’ intercooler pipes, alternator OAP pulley, Purflux filter - all changed. Using premium diesel or additive every other tank. Engine runs smoothly, no vibrations, which makes me think injectors are in acceptable condition. Turbo in good nick, variable vanes moving freely with actuator link disconnected.

I had an AP200 for the past 3 years, handy. Currently only get a couple of DTCs related to HVAC e.g. air quality sensor and BCM e.g. driver door lock motor.

Live values for engine. Always had upstream reading of 10… for the past 2/3years. Not sure how Autel reports. Attached a report.

Long story short, a quick 10 mile drive on A road post change, mileage went up 40or so. Strangely live value still 10 for lamda reading with AP200. What changed then?




Another thought/worry I have is the BM cat converter is physically smaller than original. I may go back to original because I am sure it has changed back pressure characteristics… and probably is made cheaply. I fitted it as an experiment thinking my cat was sooty(which was confirmed with a scope camera).

Thoughts welcome on aftermarket cats and why my MPG went up but same O2 reading with new sensor.