My 1999 Honda CRV has just failed it's UK MOT test on emissions.

Results are below:

1st test:
CO 0.73 (FAIL - max is 0.30)
HC 78 (PASS - max is 200)
Lambda 1.008 (PASS - between 0.970 & 1.030)

2nd test:
CO 0.59 (FAIL - max is 0.30)
HC 76 (PASS - max is 200)
Lambda 1.011 (PASS = between 0.970 & 1.030)

Idle test:
CO 0.51 (FAIL - max is 0.50)

So the only thing it failed on was the CO readings, everything else looks fine.

It's been fully serviced in the last 4 months, including oil, all filters, cambelt, waterpump, and coolant change.

What would you recommend I start by checking?

3 Answers 3


The Lambda readings are slightly lean but within the acceptable range for normal catalytic converter operation. The CO is in the normal output range for an engine that is running normally and within acceptable fuel mixture limits. That it to say that the engine appears to be running normally and no repairs to it would improve the exhaust gas output makeup. Based on the readings provided it appears that the catalyst is being fed gasses in amounts and at a mixture that it should be able to convert but that it is not able to do so. Therefore the next step I would make would be to replace the catalytic converter and then retest the system.

  • How would an exhaust leak affect this? Would be great if you could add something in your answer about exhaust leaks before and after the cat.
    – DizzyFool
    Commented Mar 22, 2017 at 11:01
  • @DizzyFool Exhaust leaks would lean the mixture (lambda) at the tailpipe as measured by the gas analyzer portion of the MOT test. Since it this within normal limits leaks are unlikely in this case. Commented Mar 22, 2017 at 15:25
  • Thanks @FredWilson - a new catalytic converter fixed it :-) Commented Mar 30, 2017 at 8:43

SmogTips.com lists a few things that you should check:

1. Air Filter

A dirty air filter will disturb the normal air / fuel ration and result in less than optimal fuel efficiency, consequently resulting in higher CO emissions.

2. O2 Sensors

The computers in your car are hooked up to the Oxygen Sensors. The computer adjusts the amount of fuel going into the engine based on the amount of oxygen in the air registered by the oxygen sensors. If the O2 sensors are giving a faulty reading, the computer will deliver less than the optimal air / fuel ratio and you could be putting out more CO.

3. MAP Sensor

This is another component that sends information to the central computer of the car. It tells the computer how strong a vacuum is being generated during the engine's intake stroke. If it is defective, it will not report the information correctly, and the fuel / air ratio will be messed up.

4. Throttle Position Sensor

If the throttle plate (usually connected to the gas pedal via a cable) is being detected in the wrong position, the engine will send more or less gas into the engine than it ought to, resulting in a less than optimal fuel / air ratio.

5. Engine Coolant Temperature Sensor

When the engine is colder, it requires more fuel. Hence, if the engine "believes" the coolant to be a different temperature than it is, it will put in more or less fuel than required, resulting in a faulty air / fuel ratio and potentially more CO emissions.

Hope this helps!


I had a similar experience with my 98 Mazda 626, slightly lean Lambda with too much CO in the exhaust. One thing that happens occasionally is that crud ( carbon, sulfur, etc. ) builds up in the catalytic converter ( and on the spark plugs ) especially if you do a lot of low RPM City driving. What I did was to take a drive in low gear for about 10 or 15 minutes and kept the RPM above 3500 for that time. High RPM driving creates higher combustion temps and higher temp exhaust which will burn that crud out of the system and you may be able to pass the emissions test after that. It happened to me two years in a row and I used this solution both times to pass the test.

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