Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing failed MOT on emissions

Hi - bit of advice. My 2002 +8 failed its MOT on emissions - the lambda sensor. Right bank was 1.07, but left bank was 1.1something

Diagnotics says the left bank O2 sensor is probably broken - reading around 5v instead of below 1v?

I'm getting worried because they're saying both cats are much hotter at the front than the back and don't seem to be working quite properly.

So - anyone any experience of this, and what to do, and whether replacing that O2 sensor will suddenly stop the ECU trying to over compensate?

Cheers, Matt

Matt, this may be of assitance.

Here is a quote from Rick's article: -

"How does an O2 sensor work?
An Oxygen sensor is a chemical generator. It is constantly making a comparison between the Oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no Oxygen in the exhaust manifold, a voltage is generated. The output of the sensor is usually between 0 and 1.1 volts. All spark combustion engines need the proper air fuel ratio to operate correctly. For gasoline this is 14.7 parts of air to one part of fuel. When the engine has more fuel than needed, all available Oxygen is consumed in the cylinder and gasses leaving through the exhaust contain almost no Oxygen. This sends out a voltage greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra Oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts. Usually the output range seen seen is 0.2 to 0.7 volts. The sensor does not begin to generate it's full output until it reaches about 600 degrees F. Prior to this time the sensor is not conductive. It is as if the circuit between the sensor and computer is not complete. The mid point is about 0.45 volts. This is neither rich nor lean. A fully warm O2 sensor *will not spend any time at 0.45 volts*. In many cars, the computer sends out a bias voltage of 0.45 through the O2 sensor wire. If the sensor is not warm, or if the circuit is not complete, the computer picks up a steady 0.45 volts. Since the computer knows this is an "illegal" value, it judges the sensor to not be ready. It remains in open loop operation, and uses all sensors except the O2 to determine fuel delivery. Any time an engine is operated in open loop, it runs somewhat rich and makes more exhaust emissions. This translates into lost power, poor fuel economy and air pollution. The O2 sensor is constantly in a state of transition between high and low voltage. Manfucturers call this crossing of the 0.45 volt mark O2 cross counts. The higher the number of O2 cross counts, the better the sensor and other parts of the computer control system are working. It is important to remember that the O2 sensor is comparing the amount of Oxygen inside and outside the engine. If the outside of the sensor should become blocked, or coated with oil, sound insulation, undercoating or antifreeze, (among other things), this comparison is not possible."

Thanks - that's interesting. So lean = < 0.45v Rich > 0.45v

I can't beleive it's running _that_ rich to push the voltage up as high as 5v, so that suggests a problem with the sensor.

However, let's assume that it is a "true" reading (or at least in the right direction). That suggests a (very) rich mixture in that bank. Would a rich mixture damage the cats over time?

I've only done about 1,500 miles in the last 12months. Is this relevant? And the battery went flat over winter - meaning loss of power to the ECU - so has something been "reset" somewhere that might cause this?

The hydrocarbon and CO2 readings were fine, can anything be inferred from this?

My understanding is that excessive heat (unburnt fuel) will damage the cats over a period of time.



ECUs do learn, so a reset may have effected some aspects of the engine managment (fuel to air ratio); however, they generally adapy quite rapidly after a reset.

As a matter of interest, how far is the MOT centre from you house and did they MOT the car as soon as it arrive - or was it left to cool and then the MOT'd?

I have a car that reads high unless it has been given a good run and not allowed to cool down before the MOT.

Well, it got thru last year, and hasn't done many miles. So, probably OK...



About 15/20mins - with traffic at the end, so it had the electric fan cutting in before it got laid up. It was being emissions tested an hour after it arrived - since I turned up to collect it at that point So, although it could've been made a bit hotter (ie done emissions immediately) there was no starting from cold.

Are you sure the 5v your getting at the O2 sensor isn't just the supply voltage from the ECU, as these run at 5v !

Get the faults/"P" codes read in the ecu, if it says the oxegen sensor is open circit/short to positive change the sensor,
give it a good thrash (low gears, high rpm) down the road for 5-10 miles and take it back for a retest.

Unburnt fuel in the cat will wreck it in a very short period of time.