Just replied to a similar topic:-
Sounds as if you need to take it to a Citreon dealer and they will connect it to their diagnostic machine whereby they will get a "Fault Code"
Almost certain to be a Lambda Sensor, one of the many sensors that are fitted to cars nowdays. Expect a bill of around £60 just for the dealer to find the fault code.
(OXYGEN (O2) SENSOR. Used on both carbureted and fuel injected engines since 1981, the oxygen (O2) sensor is the key sensor in the fuel mixture feedback control loop.
Mounted in the exhaust manifold, the O2 sensor monitors the amount of unburned oxygen in the exhaust. On many V6 and V8 engines, there are two such sensors (one for each bank of cylinders).
The O2 sensor generates a voltage signal that is proportional to the amount of unburned oxygen in the exhaust. When the fuel mixture is rich, most of the oxygen is consumed during combustion so there is little unburned oxygen in the exhaust. The difference in oxygen levels between the exhaust inside the manifold and the air outside creates an electrical potential across the sensor’s platinum and zirconium tip. This causes the sensor to generate a voltage signal. The sensor’s output is high (up to 0.9v) when the fuel mixture is rich (low oxygen), and low (down to 0.1v) when the mixture is lean (high oxygen).
The sensor’s output is monitored by the computer and is used to rebalance the fuel mixture for lowest emissions. When the sensor reads "lean" the PCM increases the on-time of the injectors to make the fuel mixture go rich. Conversely, when the sensor reads "rich" the PCM shortens the on-time of the injectors to make the fuel mixture go lean. This causes a rapid back-and-forth switching from rich to lean and back again as the engine is running. These even waves result in an "average" mixture that is almost perfectly balanced for clean combustion. The switching rate is slowest in older feedback carburetors, faster is throttle body injection systems and fastest in multiport sequential fuel injection.
If the O2 sensor’s output is monitored on an oscilloscope, it will produce a zigzagging line that dances back and forth from rich to lean. Take a look at the waveform on the opposite page - that’s what a technician wants to see when he checks the O2 - think of it as a kind of heart monitor for the engine’s air/fuel mixture.
O2 Sensor Strategies: Unheated one- or two-wire O2 sensors on 1976 through early 1990s applications should be replaced every 30,000 to 50,000 miles to assure reliable performance. Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles. On OBD II equipped vehicles, the recommended replacement interval is 100,000 miles. The O2 sensor’s responsiveness and voltage output can diminish with age and exposure to certain contaminants in the exhaust such as lead, sulfur, silicone (coolant leaks) and phosphorus (oil burning). If the sensor becomes contaminated, it may not respond very quickly to changes in the air/fuel mixture causing a lag in the PCM’s ability to control the air/fuel mixture.
The sensor’s voltage output may decline giving a lower than normal reading. This may cause the PCM to react as if the fuel mixture were leaner than it really is resulting in an overly rich fuel mixture.
How common is this problem? One EPA study found that 70 percent of the vehicles that failed an I/M 240 emissions test needed a new O2 sensor.
John, February 2008
