Code P0059: Definition and Explanation
The P0059 trouble code is a generic powertrain code applicable to most vehicles from 1996 onward (Ford, Chevrolet, GMC, Mazda, etc.). It indicates that the Powertrain Control Module (PCM) has detected an abnormal resistance in the heater circuit of the upstream oxygen sensor (O2 sensor) located before the catalytic converter on bank 2 (the bank that does not contain cylinder 1).
Role of the HO2S Sensor and Its Heater System
The oxygen sensor (HO2S) measures the proportion of oxygen in the exhaust gases. To become operational quickly after a cold start, it is equipped with an internal heater. The PCM supplies battery voltage (min. 12.6 V) to this circuit via a fuse or relay until the engine reaches its operating temperature. Code P0059 is triggered when the PCM detects a resistance in this circuit that is outside the specified range.
Symptoms of Code P0059
It is recommended to address this issue promptly to avoid damaging the catalytic converter. Symptoms include:
- ⛽ Poor fuel economy
- 🌫️ Black smoke from the exhaust (rich mixture)
- 🔧 Hard cold starts (lean mixture)
- ⚠️ Malfunction Indicator Lamp (MIL) illumination
- 📟 Possibility of other related trouble codes
Possible Causes of the Fault
The main causes of a P0059 code are:
- 🔌 Faulty oxygen sensor (most common cause)
- 🔋 Blown fuse or damaged fuse connection
- 🧵 Damaged, cut wiring, or corroded/disconnected connector
- ⚡ Faulty control relay
- 💻 Rare PCM failure
Diagnosis and Solutions
Before starting, consult your manufacturer’s Technical Service Bulletins (TSBs) to check if the problem is a known issue.
Recommended Diagnostic Steps:
- Visual Inspection: Inspect the sensor’s wiring and connectors, especially near hot (exhaust manifold) or sharp components.
- Fuse Check: Test all relevant fuses and fuse connections under load using a digital multimeter (DVOM).
- Code Reading: Use an OBD2 scanner to read all trouble codes and freeze frame data. Clear the codes and perform a road test to see if P0059 returns.
- Sensor Test: With a cold engine, check the scanner’s live data stream. The heater circuit voltage should be close to battery voltage. Use a multimeter to test the sensor’s resistance (refer to manufacturer data for correct values).
- Wiring Check: Disconnect the connectors and test the heater circuit wires for continuity and the absence of a short to ground.
Pro Tips:
- A repeatedly blown fuse often indicates a short to ground in the heater circuit.
- After repair, drive the vehicle to complete the OBD2 drive cycles and verify that the fault does not return.
Conclusion and Call to Action
Code P0059 often points to a faulty oxygen sensor. A methodical diagnosis with the right tools helps identify the exact cause and avoid unnecessary, costly repairs.
🔧 Not comfortable performing these steps? Leave the diagnosis to a professional mechanic. Use our trusted garage finder service near you to get a fast, free quote.