Car Clinic: Do I have to replace my Toyota RAV4's faulty lambda sensors?

Your car clinic expert

INSPECTOR GADGET
Dave Pollard has written several Haynes manuals and has tested just about every car-related accessory.

Q. The engine management warning light has lit up on my Toyota RAV4 and my garage says the lambda sensors need replacing. I’m told faulty sensors could affect fuel economy but are not a safety issue. Does it mean the car will fail its MoT test and are there long-term implications if I don’t fix the sensors? My garage says there are four, which seems excessive. 

MR, Bristol

A. Lambda, or oxygen, sensors are there mainly to measure whether the air/fuel ratio in the engine is correct. If the mix is too rich (there is too much fuel), it won’t be long before the catalytic convertor is damaged — and of course you’ll be using more fuel. If the mix is too lean (too little fuel), the pistons and other parts of the engine could be damaged.

One oxygen sensor sits in the exhaust downpipe, which goes from the engine to the catalytic converter. Because the RAV4 has two downpipes (one for each pair of cylinders), it needs two sensors. These sensors send information to the engine management system, which is constantly adjusting the fuelling to get the best from the engine while keeping emissions down.

Most cars also have an oxygen sensor after the catalytic converter to help gauge converter efficiency. Again the RAV4 has two.

If any one of these sensors is faulty it needs addressing sooner rather than later to avoid causing potentially more expensive damage. The illuminated warning light itself will not affect the MoT, but a faulty sensor could mean your car fails its exhaust emissions check, which means it will fail the MoT. It is highly unlikely all four sensors have failed simultaneously. Your garage should be able to discover which one is at fault fairly easily, usually with electronic diagnostic equipment. DP