GM 13 = OXYGEN SENSOR CIRCUIT OPEN

• engine running at least 2 minutes
• coolant temperature at least 50 deg C (122 deg F)
• O2 voltage not fluctuating (i.e steady between 350 and 550 mV)
• TPS signal above idle
• all above conditions met for 60 seconds

Typical causes may include:

1. Defective or degraded O2 sensor
2. Deposit-contaminated O2 sensor (running leaded fuel, RTV silicone deposits etc)
3. Corroded/defective O2 sensor connection
4. Defective sensor ground circuit
5. Defective connection at ECM
6. Defective ECM

NOTE: If codes 21 and/or 22 are also present cause ‘4’ above should be checked first.