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2006 Toyota Tacoma 4.0L V6: P0306, P0420, P0430 Faults & Strange CAT Temperature Fluctuations That Resolve After Key Off/On Cycle

Model: Bizarre CAT Temp readings Fault Code: P0306 Posted: 2019-10-19 20:11

2006 Toyota Tacoma 4.0L V6 Engine Issue Vehicle: 2006 Toyota Tacoma 4.0L V6 Symptoms: - Cold start with multiple cylinder 6 misfires (occasional misfires in other cylinders) - Exhaust Gas Temperature (CAT) readings are erratic and significantly inaccurate during initial startup - Engine remains in OPEN LOOP mode for extended periods despite reaching normal operating temperature, even under load or deceleration - CAT S1 temperatures rise to ~225°F then drop sharply to 55°F within minutes; remain low until the key is turned off - CAT S2 temperatures fluctuate between negative and single-digit values (as low as -8°F), showing instability during initial operation - Coolant temperature increases steadily without spikes or irregularities - Intake air temperature remains stable - Stoichiometric (ST) and Lean/Tailpipe (LT) fuel trims are non-responsive - Engine runs rough, lacks power, and exhibits multiple misfires while stuck in OPEN LOOP — this persists even when engine reaches normal operating temperature Critical Observation: After turning off the ignition and restarting the vehicle, all symptoms resolve instantly: - LOOP status transitions from OPEN to CLOSED within seconds - Fuel trims stabilize (within ±10%, typically single digits) - CAT temperatures begin rising into realistic ranges - Misfires cease, engine runs smoothly with full power and normal idle This behavior suggests a transient fault in sensor data or ECM logic during startup. The issue does not appear to be related to engine temperature or load conditions. Occasional P0606 (ECM memory error) has been observed but is rare and not consistent. I am using OBD Fusion with extensive logging capabilities, allowing detailed analysis of real-time data. Key Questions: 1. Are CAT temperature readings from actual sensor measurements or an ECM-derived estimate? 2. If derived by the ECM, which sensors (e.g., S1B1, S1B2, S2B1, S2B2) does it use for calculation? 3. Is there a possibility of faulty O2 sensor heater circuit behavior during startup that causes incorrect data flow to the ECM? 4. Could a failing rear O2 sensor (B2 S2) be contributing to P0420 and P0430 codes, even if it doesn't directly control open/closed loop? I am seeking expert insight into whether this is an issue with: - Sensor data integrity during cold start - ECM heater circuit protection logic - O2 sensor heater current or resistance changes over time (especially on a 13-year-old vehicle) - Or a software-level glitch in the control algorithm that resets upon key-off/restart Any technical guidance, diagnostic suggestions, or known fixes for this specific combination of codes and symptoms would be greatly appreciated.

Related fault codes
P0306P0420P0430
Comments (6)
Anonymous 2019-10-20 09:41

Can you monitor the A/F sensor heater current/voltage during startup and compare the first ignition cycle to the second? This may reveal if heater circuit issues are causing erroneous O2 sensor readings.

Anonymous 2019-10-20 10:16

Initial Start (Open Loop, poor performance): - A/F (B1 S1): MAX 3.30V, MIN 2.95V, AVG 3.29V - A/F (B2 S1): MAX 3.39V, MIN 2.02V, AVG 3.30V - O2 Sensor (B1 S2): MAX 0.92V, MIN 0.02V, AVG 0.43V - O2 Sensor (B2 S2): MAX 0.04V, MIN 0.02V, AVG 0.02V Secondary Start (Closed Loop, normal operation): - A/F (B1 S1): MAX 3.55V, MIN 2.56V, AVG 3.21V - A/F (B2 S1): MAX 3.56V, MIN 2.73V, AVG 3.22V - O2 Sensor (B1 S2): MAX 0.82V, MIN 0.02V, AVG 0.60V - O2 Sensor (B2 S2): MAX 0.88V, MIN 0.02V, AVG 0.61V The data shows that the B2 S2 O2 sensor readings are abnormally low during initial startup. This suggests a potential fault or delayed response in the rear O2 sensor. Why would simply turning off and on the ignition resolve this? Could it be related to heater circuit reset or sensor calibration?

Anonymous 2019-10-20 11:24

The ECM monitors the O2 sensor heater circuit current. It may shut down the heater circuit if it detects overcurrent to protect internal transistor drivers. During first startup, an overcurrent condition might trigger a shutdown, leading to inaccurate sensor readings. On subsequent restarts, the heater resistance may have increased due to aging or thermal effects, allowing normal current flow. This can be tested with a clamp-on ammeter on the O2 sensor heater circuit. Note: The rear O2 sensor (B2 S2) is 13 years old and could be failing — it's an affordable place to start. If confirmed faulty, it may explain P0420/P0430 codes even if not directly affecting open/closed loop operation.

Anonymous 2019-10-21 22:33

Thank you for the helpful suggestions. I will perform these tests and update with results as soon as possible.

Anonymous 2019-10-22 22:02

Hi cj1 or any experts — can you confirm whether the CAT temperature readings in OBD Fusion are derived from actual sensor data, or if they're an ECM-calculated estimate? If it's a calculation, which sensors (e.g., S1B1, S1B2, S2B1, S2B2) does the ECM use as input? For example: - S1B1: 1260 - S1B2: 1260 - S2B1: 1112 - S2B2: 1112 Since S1B1 and S1B2 are identical, and S2B1/S2B2 match, this suggests the ECM may use one of these pairs for temperature estimation. But how does it derive CAT temp from O2 sensor data? Any known algorithm or source?

Anonymous 2019-10-23 10:17

I don't have a definitive answer on how CAT temperatures are calculated, but I suspect the ECM uses O2 sensor signals as input — likely S1B1/S1B2 and/or S2B1/S2B2 data to estimate exhaust temperature. This is a common method in V6 engines, though the exact algorithm isn't publicly documented. Testing with actual heater circuit current and sensor voltage during startup could clarify whether hardware or software is at fault.