MOT Fail - erratic lambda readings
 

Hi Guys - I hope you can help.

My 2000MY Impreza Turbo Classic with 110K miles has just failed its MOT on emissions. Could anyone please suggest why the lambda readings should be so far adrift from each other on the fast idle test, and what the cure might be?

The MOT Centre suggested faulty MAF but the car is running sweet as a nut, with normal mpg.

Many thanks

Pete
PS sorry about the formatting - the first figure on each line is the required limit - the second figure is the test result


Here are the readings:

DESCRIPTION Limits Actual Value

Engine Oil Temperature Min 80 Deg C 95 Deg C OK

Fast Idle Test: FAILED
Engine Speed: 2450-3050 rpm MANUAL CHECK Pass
CO: <= 0.300% -0.004% Pass
HC: <= 200 ppm 2 ppm Pass
lambda: 0.970-1.030 2.741 Fail

Second Fast Idle Test: FAILED
Engine Speed: 2450-3050 rpm MANUAL CHECK Pass
CO: <= 0.300% -0.006% Pass
HC: <= 200 ppm 1 ppm Pass
lambda : 0.970-1.030 1.083 Fail

Natural Idle Test: PASSED
Engine Speed: 750-1050 rpm MANUAL CHECK Pass
CO: <= 0.500% 0.371% Pass

Overall Result: Exhaust Emissions Test FAILED

are you running a standard exhaust system have you recently added any fuel additives alwasy a good idea to take for mot after a run

The car is bog standard, with no additives in the fuel.

I did 100 miles on Thursday on country A roads and motorway. Today (Saturday) I drove 5 miles to the MOT test centre and they did the MOT within the next 45 minutes.

Pete

No ideas anyone??

I cleaned the MAF with carb cleaner today, but I don't know whether it has made any difference as the car was running perfectly anyway!

Pete