Us technicians need to work out the cause of a fault from what the customer and his car tells us. Usually, the car is running rough or stalling. Perhaps it has a MIL light on and is reading some fault codes. Very often we simply replace parts – but this only masks the initial problem; we’ve not dealt with the actual fault, just the effects.

The oxygen sensor is very often replaced but usually without need. Most of the time the sensor is simply telling us there is a fuelling issue which the ECU can no longer control and therefore needs diagnosing correctly. Make sure that you verify the fault codes then test, analyse and only then – replace the part.

A commonly seen fault code is ‘P0420 Cat Inefficient’. Catalytic convertors have to be fitted to all petrol vehicles and measured with lambda sensors before and after the catalyst. As you know, cat failure can be accelerated when a misfire occurs. Always make a full check to ensure that the cat has not been damaged, then clear out all fault codes, road test and check ‘mode seven’ pending codes in EOBD.

Think of it this way:how many of us has replaced an ignition coil but has not given a second thought to the damage which has been caused to the cat? If you have a car like this, you can be sure that it will be back in a few weeks later displaying the same fault code.<

Non-specific code

Also look out for ‘P0300 Misfire’. This is a general misfire fault code where the ECU cannot necessarily tell where the problem lies. Is it too much or too little fuel being supplied, poor or no spark, bad compression, a timing issue, or a problem with either the belt or the chain? We need to diagnose further. One place which is very often overlooked when investigating fuelling issues is the EOBD. Check the live data and review the Short Term Full Trim and Long Term Fuel Trim readings. As you might remember from school, three points of the four-stroke cycle are core ingredients for good combustion, namely fuel, ignition and compression. Everything must be in the right order as a misfire could occur if any one of these ingredients is missing.

When the ECU can identify a cylinder it will give us a code reading of either P0301, 02, 03, 04 - where the last digit will relate to that specific cylinder - P0301 means an issue within cylinder 1, for example.

The fault codes ‘P0420 Cat Inefficient’ and ‘P0300 Misfire’ are linked. Over a period of time if un-burnt fuel enters the exhaust system it will damage the cat, which if not diagnosed in the first place will lead to a repeat visit, a big bill and a very angry customer!<

Bad vibrations

Keep an eye open for ‘P0325 Knock Sensor’. This senses knocks and monitors vibrations in the engine. When this code is posted do not simply replace the sensor but think mechanical - pulleys, oil level and so on. In a recent issue I talked about dual mass flywheels. I have come across vehicles that have posted the P0325 fault code for a knock sensor while in fact the issue was a dual mass knocking causing the vibrations - the ECU was retarding the timing to protect the system.<

Running lean

The next code that I often see is ‘P0171 System Too Lean’. The first question to ask here is what causes a vehicle to run lean? Is there any unmetered air, split or missing vacuum pipes or split trunking?

The lambda sensor measures the gases out of the engine; when there is too much oxygen, the O2 sensor measures the mixture as lean and reports this back to the ECU. The ECU will then command more fuel through the fuel trims. If there are leaks in the system too big for the ECU to ‘fix’ then the ECU will stop trying to fix the fault and will set a code.

How is the mixture adapted? The ECU will command more or less fuel when in closed loop and also adapts the fuel trims to assist with the correct fuelling. Closed loop fuelling is where the ECU and lambda are working together to adapt the fuelling accordingly.

The lambda sensor measures oxygen as it can’t measure hydrocarbon (HC). The engine draws in the fuel and air and mixes them together at 14.7 parts of air to one part of fuel into the cylinder where they are compressed and detonated. When combustion happens most of the air and fuel is burnt and this process pushes the piston down the bore – namely the ‘power stroke’.