Checking Lambda Control; Irregularities Of The Engine After Cold Start - Skoda OCTAVIA MK1 Service Manual

Checking lambda control

The lambda probe compares the oxygen content of the air with the residual oxygen in the
exhaust gas and supplies a voltage signal to the control unit.
The voltage signal "mixture too rich (little residual oxygen)" is at approx. 0.7...1.0 V.
The voltage signal "mixture too lean (a lot of residual oxygen)" is at approx. 0.0...+0.3 V.
A voltage jump from 0.7...1.0 V to 0.0..+0.3 V or vice versa takes place at the transition from
"rich" to "lean" and vice versa (λ = 1.0).
In view of the steep voltage jump the lambda control is not able to maintain constant the
ideal mixture composition which corresponds to λ = 1.0. The control constantly fluctuates
back and forward between the "too lean" and "too rich" states.
The lambda control (also designated as lambda controller) consists of the actual lambda
probe and a specific software which is stored in the engine control unit. Depending on the
measured values of the lambda probe, the engine control unit determines the values for other
actuators, e.g for the throttle valve control unit.
If the voltage change does not occur, or if it is too slow, the following faults may have
occurred:
t Slots or holes in the probe head are clogged up.
t The probe was overheated.
t Transition resistance in the signal line or in the earth cable.
t Probe too cold, probe heating not operating.
The lambda control is switched off (engine control unit has detected faults at the injection
t system: interrogate fault memory
group °099 → Chapter).
Probe damaged by contact spray or similar (Because of the thermal fluctuations and the
t capillary effect the contact spray is sucked into the probe via the fine hollow spaces in the
electrical lines).
Probe damaged by silicone vapours (When using silicone sealants traces of silicone particles
t are sucked into the engine. These silicone particles are not combusted and damage the
lambda probe).

Irregularities of the engine after cold start

Special tools and workshop equipment required
t Vehicle system tester -V.A.G 1552 - with cable -V.A.G 1551/3-
Test condition
No fault must be stored according to the lambda control, if necessary eliminate fault or
l
erase fault memory.
The time which passes until the control (necessary probe temperature approx. 300 °C) is
→ Chapter as well as read measured value block, display
220