I'd seen both of those linked pages, so I'm not too worried that there is a catastrophic failure on the way. It's just annoying when the warning lamp comes on and I have to check it out.
I'm sure the codes relate to lambda in this engine; 2.0 TS. Multiecuscan shows them both as multiplicative fuel correction, p1177 for bank one (cyl 1 & 4) and p1178 for bank two (cyl 2 & 3) and the more detailed description seems to be that the ECU has run out of range to correct the mixture. As the multiplicative figure was 1.21 I was taking this as being a +21% correction, so making the mixture richer by 21%, and that the acceptable range was maybe +20% hence the error reporting.
But, the engine does run fine, starts and idles smoothly whether cold or hot, pulls strongly and smoothly. It's not drinking fuel or misbehaving in any other way; just the warning lamp with these associated codes.
I've put some injector cleaner in the tank in case it's a blocked injector making the mixture appear lean, but it'll take time for that to have any effect I guess.
The only other thing I can think of is an air leak after the MAF, or a failing MAF. I've tried to note the conditions when it comes on, as I can't take my laptop with MES on the drive to work. The warning lamp usually seems to come on after a prolonged period sitting idling in traffic, with the engine up to temp. Pull away when the queue clears, and hey presto, warning light on light throttle around 2000-2500rpm. Sometimes this is only 20-30 miles after reading the codes and resetting. But do a 100 mile trip down the motorway and it doesn't come on.
I've seen elsewhere that vacuum leaks at more pronounced at idle than at revs, as the amount of leaking air is constant and so a greater proportion of the air going into the cylinders at idle. The extra air is unmetered but the lambdas spot it and tell the ECU things are lean, which responds by making it richer, and eventually there is no more room to do that, so light on. On more throttle the extra air is a lower proportion of the total so doesn't lean the mixture as much.
Well, that seems to be the theory anyway. I could have misunderstood completely of course!
I may try removing the intake ducting between the MAF and throttle body to check for leaks. Plug the ends and dump it in a bucket of water and look for bubbles.
It's all fun anyway!