A 2003 Mondeo found its way to us having already been to two different garages. The car was hard to start from cold and it was smoking, though when it did start, it ran without hesitation. The first garage replaced the knock sensor and the second garage had re-coded number 4 injector by using CYL 4 on the scan tool. When this didn't work it was replaced. CYL 4 was again chosen on the scan tool to recode the injector and once again, it failed.

Cylinder vs firing order

What the previous garage had actually done was re-coded injector 2, hence the warning light remaining on. Re-coding using CYL 2 on the scan tool cured the customer's problem. To recap, you have to understand that the cylinder number and the firing order are not the same. For example, on the Mondeo, the orders are thus:

Cylinder order   1              2              3              4

Firing order       1              3              4              2

Confusing codes

Injector codes found on the rocker box might be incorrect if the injectors have been changed and can add to the confusion between cylinder order and firing order, so make sure you only use the codes that are on the injectors. Also be aware that some replacement injectors are narrower than the originals and will need the seal replacing on the camshaft cover, otherwise, an oil leak will develop.

A very clear performance indicator for common rail injectors is the diesel return-flow (spill rate). A defective injector has a significant higher or lower return-flow compared with a good injector. We must always re-check injectors whenever one injector has been replaced. This is in case a second injector is weak and can cause problems either at the time of diagnosis - which can make you think that the new injector is still faulty - or worse still, when a customer picks up the car, only to bring it straight back again.