Occasionally the old girl seems to "cough" or splutter when under light load in any gear but it's very prevalent in 4th as the entire car shunts back and forth as it misfires.

idles buttery smooth.

Under hard acceleration its fine..

Nanocom has found these faults.

cheers.

I means what it says. It adapts the mixture to keep it within pre-set limits but if it exceeds those limits it brings up the fault. If it was there all the time I'd say the bank 1 lambda sensor is dead but as it appears to be intermittent, I suspect you are looking at a dodgy connection or a sticking fuel injector. If a sticking injector I would expect to also see a P030x code to identify which cylinder.

P1171 HAS SHOWN TOO..

HMM

P1171 Oxygen Sensor System Too Lean Fault Banks A & B

So that is also pointing towards lambda sensor(s). A lean mixture gives 0V (rich gives 1V), so a dead or disconnected sensor will also show as a lean mixture. What readings did it give on emissions at the MoT? On petrol it should fail the test with a dead or disconnected sensor.

Worth bearing in mind the ECU can only see sensor inputs and act on that data.

The lambdas are giving feedback about fuel mixture.

What you currently dont know, is if the feedback is correct, and the engine is actually lean, or if the feedback is incorrect because the sensor is faulty.

I may ofcourse be teaching granny to suck eggs here, but as an example, imagine the MAF is underreading (either because its faulty, or becuase a split gasket or hose is letting some air skip past the MAF), the ECU is told there is 30g/s of air when there is actually 40g. It injects fuel for 30g, the engine is super lean, lambda detects this and ecu starts increasing fuel trims trying to fix it. Eventually it reaches the trim limit because if its having to add 33% fuel theres clearly an issue, so gives a system lean fault code.

In this situation, the lambdas are completely fine and are working as expected, the fault is with the MAF(or the air leak). But the ECU hasnt mentioned the MAF, because the MAF readings are close enough to what it expects that it hasnt realised they're wrong. Folks jump in with the parts cannon and fit new lambda sensors, and then are confused when the code remains.

I'm not saying its not the lambdas, just that care is required that you dont end up lead down a path.

It could be an air leak, fuelling issue, sensor issue etc. Try to be methodical, check the basics first, vacuum leaks and whatnot.

Given its at light load, that does to me nudge towards a possible air leak, as they tend to be fairly fixed volume and thus affect low load much more than high load.

If you're getting lean error codes and are not sure whether the fault is lambda sensor of MAF related but the engine generally seems to run OK, try reading lambda voltage while doing a few 'tip-ins' (quick blips of the throttle). If the lambda readings show a temporary rich mixture just after the tip-ins it points away from the lambda sensor being at fault. The tip-in test is possible because during such tip-in's the mixture is supposed to go richer than under normal engine running conditions, i.e. near full throttle is a special condition during which mixture is supposed to get richer than the lambda sensors can measure, the lambda sensors may max out at around 0.9v.

More likely 1 fault than 2 faults. If P1171 points to both cylinder banks lean mixture, and the lean mixture readings are due to lambda sensor issues, both lambda sensors would have failed. Or just 1 MAF sensor.

I'm not saying the fault is the MAF, I've seen plenty P38's with 2 failed lambda sensors but also plenty with failed MAF or other problems.

A misfire (due to many causes) can cause lambda sensors to give lean signal, lambda sensors measure oxygen in exhaust gas, a misfiring cylinder pumps air and fuel into the exhaust but the lambda sensor doesn't notice the fuel... unless it burns in the exhaust with the excess air before it reaches the lambda sensor. Lambda sensors need to be hot to work properly so have an inbuilt heater element, if t he heater doesn't work the sensor will give correct signal when it is hot enough (heated by the exhaust gasses) and lean signal when it is too cool to work properly.

Unless the fault is gearbox related it won't only happen in 4th gear, it's just that the fault generally occurs during the engine load conditions you apply in 4th gear. To the engine 2500rpm at 3/4 throttle is the same load regardless of what gear you're in (with a few exceptions such as if rpms are accelerating/decelerating). With this in mind, if an engine fault definitely only occurs in a certain gear, on some vehicles indeed the problem could be gearbox related. On some vehicles the transmission computer can tell the engine computer to decrease torque to aid smooth gear shifting or lessen the load on transmission clutches during the shift, particularly if the throttle isn't fly-by-wire such torque reduction is often done by momentarily retarding ignition timing.

Cheers Gents much appreciated.

after a long'ish run around 20miles she's chucking up 3 lambda sensor faults now..

P0155
P0135
P0154

HMM

Very informative posts ... I got randomly the same type of errors and probably issues. Will see what my errors are, and ponder what to do, if at all, as the car runs fine - the little I use it - and consumption is not worse than the usual horrendous :-)

P0155 is a "heater not working" code
P0154 is a "lamdba sensor no activity" code

Both for bank 2.

So those indicate Bank2 lambda is kaput

P0135 is the heater for bank 1.

Probably time to treat it to a nice pair of new sensors then!