OBD-II trouble code
P0408: Exhaust Gas Recirculation Sensor 'B' Circuit High
The EGR 'B' feedback signal is reading too high — the opposite electrical fault from P0407. On a temperature-sensor design, high voltage reads as impossibly cold, and an open circuit is the fault that produces it by default: a broken wire, a disconnected connector, or a sensor that has failed open.
Quick facts
- System
- Powertrain
- Category
- Emissions / Other
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $50 – $600
- DIY difficulty
- Intermediate DIY
Browse every code in P0400–P0499, or start from the full code library.
What does P0408 mean?
P0408 is the high-input twin of P0407, and the cleanest way to read it is as the open-circuit counterpart. Both codes concern the EGR 'Sensor B' feedback channel — which on the platforms that matter most is a dedicated EGR gas temperature sensor sitting in the exhaust-recirculation path. P0407 sets when that signal voltage falls too low; P0408 sets when it climbs above the maximum the PCM expects. For a thermistor-style temperature sensor, that high voltage is read as an implausibly cold temperature, and the mechanism is worth understanding because it tells you where to look. The PCM feeds the signal pin through an internal pull-up resistor and the sensor pulls that pin down toward ground; lose the path through the sensor and the pull-up takes the pin straight to reference voltage. So an open anywhere in the loop — signal wire, sensor element, ground return — produces this code on its own, with nothing else wrong. A short to a higher-voltage source will do it too, but in this part of the harness there is far less to short against, so read the circuit as open until a measurement says otherwise.
That 'open circuit' fingerprint is what makes P0408 a wiring-first diagnosis. A broken signal wire, a connector that has corroded until it loses continuity, a backed-out pin, or a sensor element that has failed open will all drive the reading high. Because the EGR temperature sensor lives in one of the hottest spots on the engine, thermal cycling is hard on both the sensor and its connector — a connector that has baked until it loses contact is a frequent culprit. Start at the connector and the harness, not at the parts counter.
The same architecture caveat from P0407 applies: on some platforms Sensor B is a second valve position channel rather than a temperature sensor. On those, a high-pinned feedback reading points back toward the EGR valve's internal sensor, which is why the valve replacement is offered as the secondary repair path. Confirm which design your engine uses before deciding what to replace.
Driveability impact is mild and matches the rest of the EGR feedback family. The PCM defaults the EGR to a safe strategy when it can't trust the feedback, so the engine keeps running. What you lose is proper closed-loop EGR monitoring, which costs a little fuel economy and fails an emissions inspection — and leaves a developing EGR problem unmonitored until it grows.
Common causes
- Open signal wire on the EGR Sensor B circuit (the classic high-input cause)
- Corroded connector in the hot EGR zone that has lost continuity
- EGR temperature sensor failed open internally
- Backed-out or broken terminal in the Sensor B connector
- High resistance in the signal or ground circuit from heat-baked connections
- On position-channel platforms, an open in the EGR valve's second feedback element
- Harness damage near the exhaust manifold or EGR tube
- Connector left disconnected after EGR or exhaust service
Symptoms
- Check engine light on with P0408 stored
- Mild reduction in fuel economy
- Engine usually runs normally — EGR defaults safe when feedback is lost
- Possible light pinging under load on engines that lean on EGR to hold combustion temperatures down
- Failed emissions inspection
- No direct temperature-related symptom the driver would notice
Diagnostic steps
- 1.Confirm whether Sensor B is an EGR temperature sensor or a second valve position channel on your engine before testing.
- 2.Because 'circuit high' points at an open, start at the connector: inspect for heat corrosion, a backed-out pin, melted insulation, or a connector that has worked loose.
- 3.Pull all codes, including history. P0408 alone is the Sensor B open-circuit case. P0408 and P0407 both stored on the same channel is the signature of an intermittent connection that has swung to both rails rather than two separate faults, which makes flexing and wiggling the harness the highest-value test. P0408 together with valve or flow codes broadens the picture to the EGR system itself.
- 4.For a temperature-sensor design, unplug the connector before measuring resistance across the sensor terminals. This matters: measured with the circuit still connected, you are reading the PCM's pull-up in parallel with the sensor and the number is meaningless. An infinite reading on the disconnected sensor confirms a failed element; a sane resistance that changes as the sensor warms exonerates it.
- 5.Back-probe the signal wire from sensor to PCM and check for an open or high resistance along the run, flexing the harness near the manifold to catch intermittents.
- 6.Verify the sensor ground has continuity and low resistance.
- 7.If the sensor and wiring are good on a position-channel platform, the EGR valve's feedback element has failed open — replace the valve and confirm feedback tracks afterward.
Repair cost
$50 – $600
Because high-input faults are so often a broken wire or corroded connector, the cheapest outcomes are common here — a reseated connector or pigtail repair runs $50-250. A standalone EGR temperature sensor that's failed open is $25-130 in parts, roughly $90-420 installed depending on access. If your platform uses Sensor B as a second valve position channel and that element has failed, EGR valve replacement runs $300-600+. Wiring-first diagnosis is especially worthwhile on this code since an open circuit is the most likely cause.
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DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.