The heat is on........

Had a call to look at a 2.0 Renault Clio 182, it had been around a few garages and the owner was fed up with O2 sensor diagnosis. The first garage fitted an universal sensor after reading the fault codes. The second garage fitted two Renault sensors as he only fits genuine parts. The third informed the owner that he suspected the oxygen sensors!

At this point he asked for a second opinion. This is where I enter the equation. The garage had asked me if I had seen this sort of fault before. Many times I explained, so they wanted to know the cause. Many things I explained. So armed with a list of possible causes and a warning to make sure the cause was found and not the effects.

They could find nothing wrong, and asked if I could give it the one over. When I arrived the front end was off the car, they had been checking the cam timing! (a much quicker method would of been manifold vacuum I explained. Noting the car has a map sensor I said just take a look at the live data. Key on engine off it should read 1000mbar and at idle 300mbar. If these are your readings then its a sure bet the cam timing is ok. So KOEO it was 1000mbar at idle it was 1000mar. BINGO! map sensor error staring you right in the face.

A new sensor was ordered up after testing with a scope and the cause of the fuelling problem sorted. This could lead to erroneous codes for O2 sensors and miss-diagnosis.

Next day and I am asked back the car is now back together so a test drive is possible.

The symptoms are the same as before and the fault codes remain O2 bank1 sensor 1&2.

After the test drive we put it on the ramp to check the O2 sensors and the exhaust was glowing red! Now the O2 sensors need to be 350 degrees to work but this was sensor heating in the extreme. I quick scope of the front sensor show a constant 850mvolts even when the injectors were switched off on overrun. Swapping the rear to the front restored normal operation of the fuel trims and the car was transformed. Two new sensors were fitted and the car returned to its owner. Who was less than pleased that whilst the previous garages had found the effect, O2 sensor failure, the cause had been the MAP sensor failure resulting in miss fuelling and damage to the O2 sensors and catalyst.