Code
P1418
DS
P — Powertrain
Catalytic converter downstream temperature sensor short circuit to earth
Views:
UK: 1
EN: 3
RU: 3
AI status
Completed
Completed
100%
Causes
- Damaged or chafed wiring harness contacting chassis ground
- Corroded or contaminated connector making unintended ground
- Failed downstream catalytic converter temperature sensor (NTC or heated type)
- Water or debris ingress into connector or sensor causing short
- Poor or missing sensor ground/incorrect pin contact
- ECM driver fault (rare)
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Reduced engine performance or limp mode in some cases
- Failed emissions test or increased emissions
- Possible abnormal heater fuse or relay behaviour if sensor heater shorted
- Stored related catalyst or oxygen sensor codes
What to check
- Read freeze frame and freeze data with a scan tool; note conditions when code set
- Visually inspect wiring and connector to the downstream catalytic temperature sensor for damage, corrosion, oil/water ingress or pin push-out
- Unplug sensor and inspect pins for corrosion or bent terminals
- Check for continuity between the sensor signal/heater wires and chassis ground with ignition off
- Back-probe sensor connector and measure voltage/voltage drop on the signal and heater circuits with ignition ON and engine OFF (or per service manual)
- Monitor live temperature sensor values with a scan tool while warming engine to confirm unrealistic constant/low readings
Signal parameters
- Sensor type: thermistor (NTC) or temperature sensor with heater—resistance decreases as temperature rises
- Typical behaviour: signal voltage or resistance varies with exhaust temperature; heater circuit (if present) is supplied/controlled by ECM
- Short-to-ground indication: continuity from sensor signal or heater wire to chassis ground (very low ohms) with connector unplugged
- Open/short thresholds (general guidance): near 0–5 ohms = short to ground; >100 kΩ or OL = open/disconnected; sensor signal voltages typically vary between about 0.1–4.5 V depending on temperature (check factory spec)
- Heater supply: typically battery voltage when switched on and/or ECM-controlled ground return — verify per vehicle wiring diagram
Diagnostic algorithm
- Use a scan tool to confirm P1418 is current or stored and note freeze frame values and related codes.
- Perform a visual inspection of the downstream catalytic temperature sensor, connector, and wiring from the sensor back along the harness to the ECU; repair any obvious damage.
- With ignition OFF, disconnect the sensor connector. Check continuity between each sensor wire and chassis ground. If continuity is present (near 0 ohms) that wire is shorted to ground — locate and repair the short in the harness.
- With connector disconnected, measure resistance of the sensor element (if accessible) per the vehicle repair manual. Compare to specification or look for signs of internal short (very low resistance) or open circuit (very high/OL).
- With ignition ON (engine OFF), back-probe the harness connector to check for heater supply voltage and sensor reference/signal voltage as specified. If heater supply is shorted to ground or missing, trace supply fuse/relay and wiring.
- If wiring and connector check OK, reconnect sensor and monitor live temperature readings while warming the engine/exhaust. If values are fixed at ground-level or erratic, suspect sensor failure.
- Repair or replace damaged wiring/connector. If harness repair is extensive, consider replacement of the harness section with proper insulation and routing away from hot or sharp surfaces.
- Replace the downstream catalytic converter temperature sensor if tests indicate an internal short or failure.
- Clear codes and road test to confirm the fault does not return. If fault persists after sensor and harness repair, test/replace ECM only after verifying all wiring and connectors are good.
Likely causes
- Wiring insulation damaged where harness rubs against body or exhaust shield causing direct contact to ground
- Connector corrosion or bent pin creating a short to chassis
- Failed temperature sensor internal short (sensor element or internal heater shorted)
- Water entry in connector after driving through deep water
- Less likely: ECM output/driver fault
Fault status
Status
Catalytic converter downstream temperature sensor circuit short to earth detected by ECM.
Repair difficulty: Medium
Diagnostic time: 0.5 - 2.0 hours
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