Diagnosing Lambda Sensor Problems

The MEMS injection system implements a closed-loop control system which monitors the oxygen content in the exhaust gases in order to reduce exhaust emissions. Fitted to the exhaust down pipe, this sensor is the critical component in the O2 System. The lambda sensor starts working once it reaches 300C. The lambda sensor is fitted with a heater which is switched on via a relay once the engine is running to reach this operating temperature as soon as possible. Once the sensor is at operating temperature it will read low oxygen level in a rich mixture and a high oxygen level in a lean mixture. The ECU uses these readings to adjust the fuel mixture accordingly. This adjustment happens many times a second.

The SPi Mini uses a Bosch Lamba Sensor Part No. 0258003193. Do not fit an alternative as they often fail prematurely.

What does it do?

The ECU uses the lambda sensor during two specific conditions:- (a) during idle, ie. when the engine is under no load apart from keeping itself running, and (b) during part-load conditions (which we usually term 'cruising speed') where the engine is keeping the car at a constant speed. These two conditions account for the majority of the car's operating time.

The ECU will generally ignore the lambda sensor's output during three further conditions:- (c) when the car is accelerating – the ECU will spent much of its time deliberately enriching the mixture to avoid hesitation and to provide extra power, and (d) when decelerating or 'engine braking' (which we usually term 'on the over-run'), when most ECU's will shut off the fuel completely to aid economy.

Finally condition (e) is a special case – that of cold starting, where the ECU deliberately runs the engine rich all the time to promote quick starting and ease cold-running. Once everything is up to temperature, normal operation will resume. Depending on manufacturer, a special type of Lambda sensor may be specified that has an extra quick heat-up time which means as soon as rich running is not needed anymore, the lambda is ready for operation.

During conditions (a) and (b), the ECU is said to be operating in 'closed-loop mode', and in conditions (c) and (d) the ECU is said to be in 'open-loop mode'. A correctly functioning lambda sensor will ease the transitions between these two modes.

That means the ECU will be operating as designed with accurate feedback as to the combustion efficiency. If the sensor is worn or damaged, it will not be 'set-up' properly prior to transition, so hesitation may occur. Also a rough or inconsistent idle may occur, or a loss of power at cruising speed.

This is because the ECU is acting upon false information from the worn sensor, which will not be a true measure of combustion. As the resulting air-fuel mixture is inaccurate, the engine will not run properly.

How to Diagnose

With the engine running, you should see the voltage rapidly oscillate between 900mv and 100mv. The time period should be at LEAST once a second. The voltage numbers are less important as long as they go high / low.

If the voltage is stuck high, using a hose pipe clamp, gently pinch the fuel supply 'flexi' line (orange), momentaraly, until the engine alters tone, too much and the engine will stall. This gives a lean condition and the sensor should read low volts. The lambda sensor is working correctly and the problem lies elsewhere. If the voltage stays high, the sensor may be faulty, or there may be another problem, such as a faulty lambda sensor heater relay.

If the voltage is stuck low, using a hose pipe clamp, gently pinch the fuel return 'flexi' line (green), momentaraly until the engine note can be heard to alter tone, too much and the engine will stall. This gives a rich condition and the the sensor should read high volts. The lambda sensor is working correctly and the problem lies eslewhere. If the sensor voltage stays low the sensor may be faulty.