Misfire at tick-over
Q My sons 2011 BMW 1-Series F20 1.6i has a misfire at tick-over and driving at low speed but is normal at higher speeds. It has a pending code of P2195 which I believe is to do with the pre-cat oxygen sensor. I realise that it is not necessarily the sensor at fault and that something else is causing the code. I would appreciate your guidance as to the best approach to determine the actual fault. I have a Foxwell NT301 if viewing live data would help. Also, can you recommend any books or online material to learn more about diagnostic fault-finding and use of live data?
Michael Joyner
A The code P2195 is indicating that the heated oxygen sensor signal (HO2S) 1, bank 1 – is stuck at lean. This could be caused by the sensor itself, a low fuel pressure, possible injector problems or an induction leak after the air meter, allowing air into the system altering the mixture.
It is also the case with your son’s BMW that if the mixture is altered, the system can try to compensate by over injecting fuel and this can be misleading when making the diagnosis.
A CM publication on Diagnostics can be found at https://shop.kelsey.co.uk/single-issue/car-mechanics-expert-magazine/001 Car Mechanics Expert - #1 Demystifying Vehicle Diagnostics is available for £7.99 digital or a print copy can be purchased for £8.99.
As the code is indicating a fault in the O2 sensor circuit, then this should be the first check. The sensor should be the four-wire type, and two of these wires will be the heater circuit. From the details I have these may be coloured green and orange. The heater circuit is easily tested by disconnecting the sensor and using an ohm meter between the two heater circuit terminals. The reading should be somewhere between 9 Ohms to 19 Ohms – an open circuit will immediately flag-up a fault indicating the sensor should be renewed.
The sensor circuit which should be tested next is the two wires which should be coloured yellow and black. This is done using a voltmeter set to sensitive. You are looking for voltages below 1V, connect the ground wire of the voltmeter to the black wire on the sensor and the positive probe to the yellow wire. With the engine running you should see a reading of 0.1-0.9 volts. If this reading does not move but stays at one point, then the sensor has failed and needs renewing.
Another test is to check for any vacuum leak in the induction system or inlet manifold, that may be causing the lean mixture. It is also possible that an exhaust leak would give this code, but I would normally expect this to be easily found due to the leaking exhaust gasses.