P0420 and P0430 on Both Banks: The Customer Wants Converters Today
2015 V6 crossover, 3.6 L, 152,000 miles. MIL on with P0420 and P0430. Long-term fuel trims are +3 percent (bank 1) and +1 percent (bank 2). Upstream oxygen sensors switch normally between about 0.1 V and 0.9 V. Downstream sensors switch between 0.2 V and 0.8 V at roughly the same rate as the upstream sensors. Exhaust pressure measured at the upstream oxygen sensor port: 3.4 psi at 2,500 rpm. Pressure measured at a port downstream of the converter bank 1: 0.5 psi at 2,500 rpm. The engine uses about one quart of oil per 900 miles, and coolant has needed topping up twice in six months with no visible external leak. The exhaust smells sweet for about 30 seconds after a cold start. Spark plugs were changed 20,000 miles ago and all read normal. The customer wants both converters replaced today because 'that is what the last shop said.'
1. Is the converter diagnosis supported? What does the pressure data add that the O2 data does not?
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The converters are probably degraded, but they are not the root cause, and they are not the first thing to replace.
Reading the evidence:
- Downstream sensors switching at upstream rates is the classic signature of lost oxygen storage capacity. A healthy three-way converter buffers oxygen and holds its downstream sensor relatively steady at a mid-range voltage; one that switches 0.2–0.8 V at the same frequency as the front sensor has essentially become a pipe. So the efficiency code itself is legitimate.
- The pressure test adds the why. Exhaust backpressure measured upstream of the converter should stay low - in practice under about 1.5 psi at 2,500 rpm on a healthy system. 3.4 psi at 2,500 rpm is excessive.
- The pressure split localizes the restriction. A 0.5 psi reading downstream of the converter means roughly a 2.9 psi drop across the brick. That is a plugged converter, not a plugged muffler, and it explains the reported loss of power even though the customer did not volunteer it.
- Fuel trims of +1 and +3 percent argue against a lean or rich mixture having poisoned the catalyst - the engine is metering correctly.
Conclusion: both efficiency codes are real, both converters are restricted and degraded, and both will need replacement - but the oil consumption and coolant loss are upstream causes that will kill the new converters if left alone.
2. What is the correct order of operations, and how do you justify charging for diagnosis when the customer has already been told the answer?
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Order of operations, driven by what destroys a converter:
- Coolant loss with sweet-smelling exhaust at cold start. A head gasket or intake gasket seeping coolant into the combustion chamber sends silicates and steam through the brick, which is one of the fastest ways to kill a catalyst. Confirm with a cooling system pressure test held overnight, a combustion-gas detector at the radiator, and a cylinder leakdown or block test. Repair before converters.
- Oil consumption - one quart per 900 miles. Oil ash is non-combustible and physically coats the washcoat, permanently reducing the active surface. Find the path: PCV system, valve guides/seals, or rings. At minimum, correct the PCV and quantify the consumption after a fresh oil change. A new converter behind an engine burning a quart every 900 miles will code again.
- Exhaust restriction. With the head gasket repaired, re-measure backpressure before condemning the brick; some restrictions clear once the engine stops blowing coolant through it. If 3.4 psi persists, replace.
- Converters last, then verify.
Justifying the diagnostic charge: the customer's premise is that a previous shop identified the right part. Your data does not contradict that the converters are bad - it shows they are bad and that two active engine faults will destroy the replacements. The diagnosis is what protects the several-thousand-dollar parts purchase. Being able to show the 3.4 psi reading, the 2.9 psi split across the brick, and a failed combustion-gas test converts the conversation from an opinion to a documented sequence.
3. After the head gasket repair and before the converters are replaced, how do you verify converter condition rather than guessing, and what result would let you keep the existing parts?
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Verify with a combination of tests; no single one is definitive.
- Re-measure backpressure at the upstream port at 2,500 rpm. Under about 1.5 psi means no meaningful restriction and the brick may be physically intact.
- Temperature rise across the converter. Inlet versus outlet surface temperature at a steady 2,500 rpm should show the outlet hotter - the oxidation of CO and HC releases heat. A dead converter shows little or no rise; a restricted one shows a very hot inlet and a cooler outlet. Use an infrared thermometer on matched points on both sides.
- Oxygen storage test. Force the mixture rich and lean with propane or a scan-tool command and watch the downstream sensor's response time. A healthy brick takes noticeably longer to follow the commanded excursion than the upstream sensor. If the downstream sensor reacts in the same time frame as the front one, storage capacity is gone.
- Re-scan for pending codes after 40–50 miles of mixed driving including a couple of steady 45–55 mph cruise cycles that let the monitor run.
What would justify keeping the parts: backpressure back under about 1.5 psi, an outlet temperature clearly above inlet, a downstream sensor that lags the upstream sensor during propane excursions, and no P0420/P0430 after a full monitor run. In practice, given that a downstream sensor was already switching at upstream speed and the brick showed a 2.9 psi drop, expect to replace - but you will be replacing them with data behind the decision and with the two root causes already corrected, so the repair will hold.