Diagnostic: oxygen sensor Citroen C5 I

Flow chart

(1) ignition injection control unit

(2) position and speed sensor

(3) inlet manifold pressure sensor

(4) throttle potentiometer

(5) engine coolant temperature sensor

(6) air temperature sensor

(7) vehicle speed sensor

(8) oxygen sensor

(9) battery

(10) injection double relay

(11) "Static" ignition coil

(12) fuel tank

(13) fuel pump

(14) petrol filter

(15) supply rail/inlet manifold assembly

(16) fuel pressure regulator

(17) injectors

(18) canister

(19) canister discharge electrovalve

(20) throttle butterfly housing

(21) throttle butterfly housing heating element

(22) idle speed regulation solenoid

(23) diagnostic warning lamp

(24) diagnostic socket

(25) sparking plugs

Function of an injection system with a catalytic converter

The engine management ECU controls the air-fuel mixture to ensure the optimum running of the engine

The calculation of the quantity of fuel to inject is based on the following parameters

• Engine speed
• Butterfly angle
• Pressure in the inlet manifold

Corrections are applied as a function of the coolant and air temperatures

The fitting of a catalytic converter implies the optimum control of the air fuel mixture either by adjusting the injection time as a function of the information supplied by the oxygen sensor mounted in the exhaust pipe

The oxygen sensor detects the presence of oxygen (weak mixture) or the absence of oxygen (rich mixture) in the exhaust gases

The oxygen sensor operates at temperatures above 350°C which requires the fitting of a heating facility provided by an integral electric element

Oxygen sensor

(26) protection cap open to allow a flow of exhaust gases

(27) body

(28) outer protection cover (crimped but not airtight)

(29) insulation

(30) output to the fuel injection ECU

(31) heating element supply (switched + and earth)

(32) contacts

(33) heating element

(34) ceramic support

(35) porous ceramic, platinum electrodes

The oxygen sensor comprises a hollow body of special ceramic material covered with a fine platinum-based coating

The exterior of the ceramic is in contact with the exhaust gases

The interior is in contact with a pocket of air which serves as a reference

Successive heating and cooling of the oxygen sensor causes a change of volume of the air pocket

Compensation of the change of volume is achieved by the provision of an air passage across the wires then the metal terminals of the connector which, though folded, are not airtight

It is absolutely forbidden to apply grease or cleaning products to the terminals as this could introduce hydrocarbons contained in the grease or solvent into the reference air pocket

These hydrocarbons alter the state of the reference air in the sensor and therefore the mixture signal accuracy

Checking the oxygen sensor

Operational faults

The autodiagnostic warning lamp lights

Excessive fuel consumption

Cold starting problems

Engine hesitation

The engine stalls

Unstable idling

Methodology

Carry out a reading of the engine ECU fault codes

2 types of faults appear

• Autoadaptation or mixture regulation limit
• Oxygen sensor

Autoadaptation or mixture regulation limit

Front door exterior opening control handle

• Ask the owner if the vehicle has been driven with the "low fuel" warning lamp illuminated
• Check the fuse for the oxygen sensor heating

Permanent fault

• Ensure that the fuel is of the type recommended: unleaded petrol
• Check the exhaust for leaks before the catalytic converter
• Check the air inlet
• Check the canister purge electrovalve (jammed in the open position)
• Verify the ignition
• Check the fuel pressure
• Check the condition of the injectors (fouled, leaks..)
• Check the mechanical condition of the engine
• Check the oxygen sensor

Oxygen sensor

Verify

• The condition and contact of the connections of the oxygen sensor (no grease nor cleaning products used)
• The sealing of the exhaust pipe around the oxygen sensor fixing
• The oxygen sensor seal
• The oxygen sensor reference and its compatibility with the engine

Using a diagnostic tool (ELIT, SOURIAU 26A)

Select the menu: param. measuring

Engine hot, at idle: the voltage must vary between a minimum of 0 mV to 250 mV and a maximum of 700 mV to 1 V at least 1 times a minute

Select the menu: PHYSICAL MEASUREMENT

Use the cables provided for testing the oxygen sensor

Follow the diagnostic equipment program

Remove the oxygen sensor

Check the condition of the oxygen sensor wiring by pulling on each wire

The oxygen sensor for fouling

• Oxygen sensor covered in black, sooty deposits: problem of rich mixture (check the fuel pressure, pressure sensor...)
• Oxygen sensor covered in a greasy film: check the mechanical condition of the engine

Check a removed oxygen sensor

Heating resistor

Use an ohmmeter

Check the resistance between terminals 1 and 2: 1 ohm < R < 15 ohms

Earth

Check the resistance between terminal 3 and the body of the sensor

• Reference LSH 6.12.24: R < 5 ohms
• Reference LSF: R = infinity

Insulation

Check the insulation between terminals 3 and 4: R = infinity

Check the insulation between terminals 1 and 4: R = infinity

Check the insulation between terminals 1 and 3: R = infinity

Harness check

See the corresponding operation (refer to the vehicle's DIAGNOSTIC brochure)