Expert answers petrol questions

The mystery behind the trouble experienced by thousands of drivers could be solved today when test results on suspect fuel are revealed.

The problem seems to be particularly affecting oxygen sensors in vehicles - although retailers have so far insisted their petrol is not faulty.

AA technical specialist Vanessa Guyll explains what might be happening if the fuel is at the root of the problem.

WHAT IS CAUSING THE PROBLEMS?

We really don't know, but are fairly confident the problems are caused by faulty or contaminated fuel.

COULD SILICONE BE THE CAUSE?

At the moment we don't know whether the contaminant is silicone and, if it is, really have no idea whether it will cause the damage we have been seeing. The further test results by Trading Standards should hopefully prove more conclusive.

WHAT HAPPENS WHEN OXYGEN SENSORS ARE AFFECTED?

The oxygen sensor measures the level of oxygen in the exhaust gases before they enter the catalytic converter. This allows the engine management system to adjust the fuelling to make sure the mixture is correct - not too weak or too rich.

WHAT ARE THE SYMPTOMS BEING EXPERIENCED?

The oxygen sensor is damaged and is then unable to supply information to the ECU of the engine management system. In the absence of any information on mixture strength and combustion quality, the ECU adopts "Limp Home" mode which means the car still drives, but performance is restricted.

WHAT LASTING DAMAGE CAN BE CAUSED?

Once the sensor has been replaced and fuel changed we are not aware of any further damage.

HOW CAN IT BE PUT RIGHT?

Repairers will need to carry out a diagnostic check and then replace the sensor. The costs will vary with labour rates and cost of the component, but could be in the region of £200 for many cars.

WHAT SHOULD AFFECTED MOTORISTS DO?

Our advice is as follows:

1) If the engine management warning lights up on the dashboard, take the car to your dealer or garage as soon as possible.

2) If the dealer or garage suspects the fault is contaminated fuel, have a sample collected from the fuel tank and properly labelled.

3) Keep receipts from petrol stations to prove when and where the contaminated fuel was purchased. Credit card statements will also help.

4) Although the petrol station that sold you the fuel may not be directly to blame, your contract is with them for selling the fuel and it will be them who you seek reimbursement/compensation from. Contact your local Trading Standards office for detailed advice as it is likely that they will be aware of other claims.