Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Front Ho2S Heaters
The HO2S heaters are monitored for proper voltage and current. A HO2S heater voltage fault is determined by turning the heater on and off and looking for corresponding voltage change in the heater output driver circuit in the PCM.
A separate current-monitoring circuit monitors heater current once per driving cycle. If the measured heater current falls below or above a calibratable threshold, the heater is assumed to be degraded or malfunctioning.
| DTCs Sensor 1 | P0135 - O2 Sensor Heater Circuit, Bank 1 P0155 - O2 Sensor Heater Circuit, Bank 2 P0053 - O2 Sensor Heater Resistance, Bank 1 P0059 - O2 Sensor Heater Resistance, Bank 2 |
| Monitor execution | once per driving cycle for heater current, continuous for voltage monitoring |
| Monitor Sequence | Heater current monitor: Stream 1 HO2S/UEGO response test completed (2010 MY and earlier), Stream 2 and 3 HO2S functional tests completed (2010 MY and earlier), HO2S/UEGO heater voltage check completed |
| Sensors OK | Heater current monitor: no HO2S/UEGO heater voltage DTCs |
| Monitoring Duration | < 10 seconds for heater voltage check, < 5 seconds for heater current check |
| Entry Condition | Minimum | Maximum |
| Inferred HO2S 1 Temperature (heater voltage check only) | 150 °F | 1250 °F |
| Inferred HO2S 1 Temperature (heater current check only) | 250 °F | 1250 °F |
| HO2S 1/2/3 heater-on time (heater current check only) | 30 seconds | |
| Engine RPM (heater current check only) | 5000 rpm | |
| Battery Voltage (heater voltage check only) | 11.0 volts | 18.0 volts |
| Smart driver status indicated malfunction Number monitor retries allowed for malfunction > = 30 Heater current outside limits: < 0.220 Amps or > 3 Amps, (NTK) < 0.400 Amps or > 3 Amps, (Bosch) < 0.465 Amps or > 3 Amps, (NTK Fast Light Off) < 0.230 Amps or > 3 Amps, (Bosch Fast Light Off) |
| Monitor ID | Test ID | Description | Units |
| $41 | $81 | HO2S11 Heater Current (P0053) | Amps |
| $45 | $81 | HO2S21 Heater Current (P0059) | Amps |
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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