Yaris MOT Failed - Running Rich AF / Oxygen Sensor

[SB1500]

Hi there, a relative's Yaris, 1.33 petrol failed its MOT on emissions. We can also tell it's idling high and running rich. We've looked into the following sensor values on the Autel scanner but are not sure what else to check. The spark plugs are all quite dark, from how rich it is running. We're not sure if it's the MAF (we've tried cleaning that) or if it's the front AF sensor, or mid exhaust oxygen sensor.  The sensors (genuine) are around £160 each and the car is older now - so we don't want to throw parts at it unless necessary. 

Any thing we could check on each of those parts to be certain that they need replacing? Or anything else in general to check with these problems? 

 

[Mooly]

Do they look anything like this:

 

The 1.33 has problems with burning oil so that has to be the first question, is this an oil burner?

 

[Bper]

Hi Shane, the symptoms you are describing may well be caused by the front air fuel ratio AF sensor, also known as the oxygen sensor, it constantly monitors the amount of oxygen in the exhaust stream, which helps the engine computer determine how much fuel to inject for optimal combustion.A faulty AF sensor can definitely cause the symptoms you described.

High idle, a rich fuel mixture can cause the engine to idle at a higher RPM than normal. Running rich, If the sensor is sending a signal that indicates a lean condition more air than fuel, the engine computer will compensate by adding more fuel, resulting in a rich mixture, if you can't test this yourself then a mechanic can use a scan tool to monitor the sensor's output voltage. If the voltage is erratic or outside the expected range, the sensor is likely faulty and needs to be replaced.

While a faulty AF sensor is a common cause of these symptoms, it's not the only possibility. Other problems such as a dirty air filter, vacuum leaks, or failing fuel injectors can also cause a rich running condition. Hope this helps.

[SB1500]

Hi all, we got this sorted last week. It turns out that the wiring going into the front sensor, was ruptured. Upon closer inspection, the red wire inside the loom which connects it, was snapped. All the others were fine. 

Reconnecting this with a wire join, clearing the code, taking it for a drive - and it's gone. 

Had we bought a new sensor that would have been £80 - £160 down the drain and no solution.  

If you have this issue please trace back your wires and check for these 'obvious' but hidden signs first before playing part roulette. It solved this one 🙂 thanks for your replies above 

This car was accidentally crashed into a garden wall at the start of the year, slipping in ice at the bottom of a long hill.. It's been rebuilt recently so the broken wire is most likely a result of that. Everything else seems to be fine since it's been rebuilt / re-aligned.