Engine running rough

Hi Folks

Changed my back box, cat and down-pipe a few weeks back. Had problems getting sensors off and damaged one in the process. Replaced the pre cat one and all was well for a few days, then engine warning light came on.Hooked up OBD2 reader and discovered post cat sensor was faulty. Ordered up replacement but drove car for 10 days before i got it fitted. The day before i changed it the car started running rough and losing power. Even after changing sensor and erasing fault code ( which hasn't come back on ) the car is still running rough and losing power. It seemed fine on the motorway when i took it for a test drive after replacing the post cat sensor. But when driving in town its rough and sluggish. Driving me round the bend. Whats the most likely problem or problems:bang: ? :bang:

Always suspect the last thing you did. Except in this case it's not the post cat lambda! Even a duff one won't effect the running of the engine. I've been running around with a dead post cat sensor for 12 months!:omg:
Check for leaks on the exhaust manifold and down pipe. Double check the pre cat sensor.

If you have torque or similar, can you see the readings off the pre-cat sensor?
Other than that, it's back to basics. Check fuel, ignition, induction, timing...

Thanks for the reply David. Weather permitting I'll get under the car and check for leaks tomorrow as I've just come in after a night shift and a bit of o/t.Thinking about it, it is a bit noisy. Also using a bit more fuel. Readers just a cheap one from eBay so can't get any readings from pre cat.

The post cat sensor is purely to confirm the readings, it does not affect tune in anyway.

You really need a reader to see what is going on.

Thanks for the reply talkingcars. The reader I have is just a simple fault code reader. Can you point me in the right direction for a reader that can give me readings from the sensor.

Sounds very similar to when my pre cat sensor went. The car ran like a pig on tick over and was very hesitant when I accelerated.

I would check the precat sensor maybe the wire has got damaged or it is a faulty sensor, in which case you may get a free replacement.

You could try swapping the post cat sensor for the pre cat I believe they are the same.

I'll give the sensor swap a try Bob and check all the wiring. As the exhaust is new I think it would be easier to take the cat and down pipe off.

Postman87 wrote: Thanks for the reply talkingcars. The reader I have is just a simple fault code reader. Can you point me in the right direction for a reader that can give me readings from the sensor.

The two I have are quite expensive at well over £100 each but they do so much more than the cheap ones.

The first is a scangauge2 and is purely an OBD2 reader, mine lives in my ZS mounted on the steering column and giving me four extra bits of information as I drive.

https://www.scangauge2.co.uk/

The second is a pscan, it is designed for MG/Rover applications and accesses most systems in our cars. It has to be used in conjunction with a laptop but can show you live data for all the ECU parameters as you're driven.

www.pscan.eu

Postman87 wrote: Can you point me in the right direction for a reader that can give me readings from the sensor.

The cheapest way is to buy a Bluetooth OBD2 dongle and download Torque onto your android smartphone. You can then see all sorts of live data including outputs from the lambda sensors and engine coolant temp sensor etc. A faulty coolant sensor is another possibility.

Apple phones need a WiFi OBD2 dongle.

Been really busy so all I've managed to do to date is tighten everything up and it's made no difference. Ordered a wifi obd2 from Amazon so it will arrive tomorrow. Was planning to swap over the sensors Friday and see if that will make a difference. But I might just plug in the wifi obd2 first as see what comes up. Thanks for all you suggestions I'll get to the bottom of it eventually.

talkingcars wrote: The post cat sensor is purely to confirm the readings, it does not affect tune in anyway.

Is that just the case with these cars? Because it isn't the case with Nissan (Micras anyway). The post CAT sensor measures the output from the cat to ensure it is working efficiently and working together with the pre-cat sensor, adjusts the ECU. It's quite common for the post cat sensor to fail, pulling the pre-cat sensor out of spec, giving a bank 1 sensor 1 error. You change bank 1 sensor 1 and still have the error until you change bank 1 sensor 2.

I know that't not relevent here, just curious....

Nick

A while ago the wiring for the post cat probe on my ZS became damaged, I insulated the 4 broken wires and hid them in the trunking, it's passed 2 MOTs since with good emissions.