Faulty heated catalyst power switch relay or control module internal failure; Corroded, loose, or damaged wiring and connectors in the heated catalyst power circuit; Short circuit to ground or power in the heated catalyst power switch circuit; Failed heated catalyst element causing excessive current draw and overheating; Engine control module software or hardware fault incorrectly reporting overtemperature; Poor ground connection for the heated catalyst circuit causing resistance and heat buildup

DTC codes / BMW codes / P1402

P1402 · BMWHeated Catalyst Power Switch Overtemperature Condition Bank 1

P1402 on BMW vehicles indicates that the engine control module has detected an overtemperature condition within the heated catalyst power switch circuit for bank 1, which is the exhaust bank containing cylinder number one. The module monitors the power switch that controls current flow to the heated catalyst and sets this code when the switch or its circuit exceeds a safe thermal threshold, typically due to excessive resistance or a short.

SystemPowertrain

Code typeBMW specific

SeverityMedium

Fix difficultyIntermediate

AreaAuxiliary emission controls

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
A burning or hot electrical smell from the underhood or underbody area near the catalyst
In rare cases, a slight hesitation or reduced engine performance if the catalyst heating is inhibited
Possible intermittent warning light if the overtemperature condition is marginal
No driver-perceptible change in most cases because the heated catalyst is primarily an emissions aid

Common causes, most likely first

Faulty heated catalyst power switch relay or control module internal failure
Corroded, loose, or damaged wiring and connectors in the heated catalyst power circuit
Short circuit to ground or power in the heated catalyst power switch circuit
Failed heated catalyst element causing excessive current draw and overheating
Engine control module software or hardware fault incorrectly reporting overtemperature
Poor ground connection for the heated catalyst circuit causing resistance and heat buildup

How to diagnose and fix it

Connect a BMW-compatible scan tool and read all stored codes and freeze-frame data to confirm P1402 and check for related codes
Visually inspect the heated catalyst power switch relay, wiring harness, and connectors for signs of overheating, melting, or corrosion
Measure resistance and voltage drop across the heated catalyst power switch circuit to identify excessive resistance or a short
Test the heated catalyst element resistance and compare to BMW specifications to rule out a failed catalyst drawing too much current
Check and clean all ground connections associated with the heated catalyst circuit
If the circuit tests normal, reprogram or replace the engine control module as per BMW diagnostic procedures
Clear codes and perform a road test to verify the repair and ensure the code does not return

Overtemperature code misdiagnosis traps

A P1402 code often results from a poor ground connection rather than a failed power switch, so technicians who replace the switch without checking grounds may see the code return.
The heated catalyst power switch is frequently integrated into a relay module that also controls other functions, and a fault in an unrelated circuit can cause overheating symptoms in the switch.
On some BMW models, the heated catalyst is only active during cold starts, so the overtemperature condition may not set immediately and can be missed during a short test drive.
A failing battery or charging system can cause low voltage that makes the power switch work harder and overheat, so always verify battery condition before condemning the switch.
This code does not always mean the catalytic converter itself is failing; the overtemperature is in the power switch circuit, and replacing the catalyst without diagnosing the circuit is an expensive mistake.

Can I keep driving with P1402?

You can generally continue driving with P1402 because the heated catalyst is an emissions component and the engine will run normally, but avoid extended idling or repeated short trips that keep the catalyst heater active. Have the fault diagnosed soon to prevent potential wiring damage or a no-start condition if the circuit fails completely.

Tools that help

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Guidance is general and written for BMW vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.