As Chris99 says, it could be the Lambda sensors? Try transposing them and see if the snag switches from PO173 (rear cylinder) to PO170 (front cylinder). If the PO173 remains it's not a Lambda sensor at fault. Codes PO170 and PO173 are often caused by a fuel supply pressure issue with the ECU extending the pulse to make up for a low delivery pressure. Do you have the original fuel pump fitted or has it been replaced with a more reliable Walbro one? The original fuel pump is the cause of many running problems and should be replaced. You would think that a faulty pump would give problems with both cylinders but it usually just indicates one cylinder, the fuel maps are not identical front and rear.
The standard locked ECU will normally manage to operate with a free flowing exhaust without fault codes, I doubt that is the problem but you could ask your dealer to install a Stage 1 map if you have concerns. Not sure how much they charge for that these days??? Worth sorting it out if you are planning to sell as a frequent EML will not help a sale. The fuel pump parts are not expensive, a bit over £100.
Yes I will try swapping the sensors when I get a chance, they appear to be easily accessible so I presume it's easy to do this - it'll be interesting to see what the outcome is.
I don't know if I have the original fuel pump but I would assume that I do. I bought the car at 3 years old and 2,000 miles and whilst the exhaust was modified (I assume), I don't seem to have a receipt for a fuel pump. Where can I buy a Walbro fuel pump and are there any additional parts I need? Is it an easy install and is it "plug and play" or does any coding need to be done wit the MOT?
