Diag. Code 21 Main Oxygen Sensor and Heater Signal
CIRCUIT DESCRIPTION
The main oxygen sensor is located in the exhaust manifold. It indirectly determines whether the fuel
mixture is rich or lean by detecting the concentration of oxygen present in the exhaust gas.
The main oxygen sensor is provided with a heater. The heater is controlled by the ECU. When the
intake air volume is low (the temperature of the exhaust gas is low) current flows to the heater to heat
the sensor for accurate detection.
Code No. Diagnosis Trouble Area
(1) Open or short in heater circuit of main oxygen
sensor for 0.5 sec. or more (HT).
• Open or short in heater circuit of
main oxygen sensor
• Main oxygen sensor heater
• ECU
21
(2) At normal driving speed (below 60 mph and
engine speed is above 1,500 rpm), amplitude
of oxygen sensor signal (OX1) is reduced to
between 0.35–0.70 V continuously for 60
sec. or more
(2 trip detection logic)*
• Open or short in main oxygen sen-
sor circuit
• Main oxygen sensor
• ECU
*: See page TR–13.
TR–36
–ENGINE TROUBLESHOOTING Circuiit Inspection
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