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post #10 of (permalink) Old 28th February 2007
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I work for a Renault Main Dealership and we have had 9 Upstream sensor faults in 2 days. Symptoms include...Misfires, stalling, sluggish engine, and at worst almost totally undriveable. We had an 06 Laguna affected, scenic 1 and 2s as well so most of the range can be affected.

Lambda sensors
Most Renaults since around 2001 now have 2 lambda sensors. These are situated in the exhaust pipe, 1 before the catalytic converter and 1 after the cat.
The sensor in front of the cat is refered to as "Upstream" and the sensor after the cat is known as "Downstream".
The upstream sensor is responsible for the mixture adaption and ensures the engine managment runs the engine at 14.7:1 fuel/air ratio. The sensor produces its own voltage of anything between 0 and 1000 mV. 0 being Lean and 1000 being Rich. A healthy engine and sensor will give a sensor reading consisting of fluctuations between 50mV to 950mV about once or twice a second.
The downstream sensor is a monitor of catalytic converter efficiency. This should typically stay around 400 - 500 mV. It is an indication of the oxygen storage of the catalytic converter. If this signal starts to fluctuate like the upstream, it means the cat is defective.

Whatever is in the petrol is contaminating the upstream sensor making its voltage hover around 350-650 mV and not fluctuate up and down. The engine managment reacts to this by richening up the mixture to try and get the sensor to respond (ie, the car thinks its running lean). The sensor does not move and the engine runs so rich it causes running problems.
Also, the downstream sensor may drop down as low as 20 mV. This is an indication of the Catalytic converter working overtime to burn off the excess emmisions as all the oxygen is consumed burning of excess Carbon Monoxide and Hydro carbons (Unburnt Petrol)
Having a new upstream sensor fitted will cure the problem, however, if the current fuel in the tank is not diluted with good fuel, the problem is likely to return. I have not as yet had a repeat visit, but im sure we will see one!
I hope this helps anybody!
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1996 Megane 1.6 RT
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