Oxygen sensor replacement – some shared thoughts.

I replaced the pre-catalyst oxygen sensor on my Sirion 1.5 SX recently, and it was not a step I took lightly because it was not cheap. Long story short I knew something was amiss with the car because of increasing fuel consumption and a loss of power and smoothness at motorway speeds. I did a lot of reading on the subject and all indications pointed to a failing lambda sensor. I had no ECU fail codes and my decision was based on knowledge and experience of all things cars – an intelligent hunch.

I bought the car last year second-hand with a genuine 17,000 miles on the clock and I replaced the sensor at 26,000 miles. Wwhy the oxygen sensor had begun failing so soon I have no real idea. The previous owner had been by the coast so maybe a corrosive air mixture had been harsh on the sensor, or contaminated fuel at some stage had stuffed it. Who knows.

The car has two oxygen sensors – one pre-catalyst and one post-catalyst. The crucial one is the pre-catalyst. This is the one that controls the fuel air mixture and subsequently directly affects engine running/efficiency. The post-catalyst sensor is known as the “diagnostic” one and monitors the actual catalyst health and therefore has no bearing on fuel consumption based on what I have read and interpreted myself.

So armed with my new knowledge I spent an age looking for an after-market item, and my advice is save yourself some trouble and don't bother looking because they don't exist for this very rare model 1.5 SX Sirion. There are “universal” ones but for such an important sensor I bit the bullet and bought an OEM (Original Equipment Manufacturer) item at almost £300. Like I said it was a decision not taken lightly but I do a lot of mileage so something had to be done because the car was becoming unpleasant to drive all day.

However the first turn of the key rewarded me with an engine that was once again a happy thing. The difference was that obvious just at idle speed, and once up and running things got better. Motorway cruising is effortless and sublimely smooth, acceleration is guaranteed to give most other cars a wake up call from traffic lights, and above all my fuel consumption matches book figures and I can get 50 mpg with a purposeful driving style, and backing off the throttle pays me back with a genuine 54 mpg.

I hated the expense but the rewards were huge and I am happy because it has transformed the car into something special to drive once more. They say they are Japan's best kept secret.

On the subject of after-market sensors I can find no definitive articles as to whether they are any good, so I have subsequently bought a universal item (£18) for the post-catalyst position for a trial to see if it triggers any fault codes. Visually it is identical and has the correct wiring. I have not fitted it yet but I will soon. I am very curious about “universal” sensors because they are so cheap and I may buy another one for the pre-catalyst position and swap out my OEM item for a trial.

I will post again with any findings I come up with, but in the mean time if you have inexplicable poor running performance you might want to think about a new pre-catalyst sensor. Yes I know they are expensive, but if you love your car and want to save fuel knock yourself out and do it. My hunch paid off.